Introjects are real alter types, the brain takes aspects of an already formed character to project memories, experiences, and opinions onto, maybe because it believes if it were that character it would be happier or able to handle a situation better. Children often have stronger examples of the thoughts that would cause this:"If I were a T-Rex I could be stronger than [abuser]" or "That's not happening to me that's happening to [evil cartoon character] so they deserve it"
When introjects come into the picture and they begin to fully engrose themselves in the idea that they are a completely separate person with a completely separate life from you, the dissociation gets worse. It is YOUR brain telling you that you are this character when you enter a different state of consciousness. You are not literally that character nor do you actually have any of their experiences.
Alters are not different people, treating them as such even without fictives in the mix can be borderline delusional depending on how severe it is. The brain wants you to fully believe something that is not true so that it can protect itself. A delusion is a false belief or perception. Simple as that. When one of the parts of your brain fully believes that it is a fictional character that is a false perception. However, this is different from acknowledging that one of your alternate states identifies heavily with a fictional character because of [trauma, aspect of daily life, etc].
Now alters themselves are not delusions but often times alters perception of reality is not true. People with DID often have delusions that people are out to get them. These would be explained by the PTSD portion of the disorder. Delusions can be caused by a multitude of different factors and dissociation and trauma is one of them
The whole point of DID is to be able to function while being traumatized. It's hard to do that if you can always remember trauma. As a child you deny [bad thing] either it didn't happen period or it didn't happen to you. Either way you are distancing yourself from those experiences but by doing that you deny a part of you that your brain still knows about. So instead these memories become only accessible when a certain part of your brain is active. Eventually you will form opinions based on only this version of the world but it conflicts with your other more functional world views so the dissociation barriers become stronger to continue denying your own experience.
Introjects are a phenomenon that happen, but they aren't genuinely that character in anyway, it is you but the part of you that for some reasons needs the traits of that character to either hold trauma or function normally. In relation to exotrauma as you called it, that probably means the brain is using the experiences of a character to mask its own trauma while still having a place to put it. By associating its own trauma with the trauma of a fictional character it can continue to deny its own reality. DID is just a very long term and severe coping mechanism and trauma response that changes the way the brain functions. If it feels like it needs to be [character] to function, it is going to want to fully believe it is that character.
I am mostly concerned with individuals who engage heavily with other introjects or role players and who hold the belief that alters are fully separate individuals. You do not have [character] in your head, one of your dissociated states is a manifestation of [character]. That is a part of you that is disconnected from yourself and parts of objective reality that you as one individual experience.
DID is meant to make you feel like you haven't experienced certain things by putting those things in different places. Playing into the identity of an introject alter can become dangerous really fast, especially when you becomes distressed because you miss characters you have never met or can't look at yourself in the mirror.
I had an introject at one point many years ago and whenever I was in that alternate state I fully and genuinely believed that I was that character. I had false memories, I couldn't look at myself in the mirror, and I would genuinely sob because I could not meet "my friends". When I was not in that alternate state I thought it was so stupid that I was so distressed or sometimes I wouldn't even remember why I had been upset. I had to stop engaging with his streams, I had to stop engaging with his friends streams, I was delusional when I was in that alternate consciousness because my brain needed me to not associate myself with my life or experiences.
When having an introjected part causes severe distress because of personal perceptions and false beliefs it is likely because one is feeding into a disconnect from reality and oneself, which is just dissociation. It is basically dissociative induced delusions which don't sound crazy when you think about what dissociation is. It does impact quality of life and is not a desirable symptom, however having delusional thinking or delusions does not make someone morally inferior or worth less.
Introjects may be so connected to their source because the brain does need to escape from stress or needs to disavow a trauma holder. Introjects may form already severely connected to their idea of a character, however, I am mainly concerned with individuals whose introjects do not start off in a basically delusional state. I often see people trying to make their fictives feel more like [character] by constantly engaging with the media just so that their fictives develop more and actively allowing themselves (and maybe even being encouraged) to engage with what is basically roleplaying false memories with other introjects or kinnies.
Unless it is necessary to keep up dissociative barriers for the safety of an individual it is not really ideal for them to be feeding into their own disconnect from reality/themselves. Introjection is also often not full characters, it is usually the taking of certain traits that are adapted onto an existing fragment or a basis for a new part. I am not saying introjects are inherently delusional, I am saying some people engage with and actively indulge in their dissociative behaviors so much it pushes them over the edge to delusion.
I don't know about you but when I'm really dissociated I may disown my body because "it's not mine" and I consider that to be a delusion so how is an alter claiming to be a completely separate person with their own life before hand not in some way a dissociation induced delusion?
Having a fictive who you acknowledge as an alter ≠ having a fictive who fully believes they had certain experiences, have been taken from their old life, have personal connections with other characters, etc
All in all: alters are not delusions but often alters, even non introjected ones, have false beliefs or perceptions that are driven by dissociation and it is ideal to combat them to achieve a higher quality of life.