So many of the things people point out about Zeke's "coping mechanisms" or "just how he is" come across super ND to me. Zeke's ear scratching is a legitimate stim irl for a lot of people. Smoking could also be considered self-stimulatory as well. He uses his interest in baseball as a way to bond with Colt, as well as giving Eren his mit. Also, his conversation with Armin shows so much disconnect and lack of understanding the people around him.
As a child, he was an introverted people pleaser, and throughout his life, he is easily manipulated. The way that Zeke acts would be indicative to me of more than only PTSD irl. A lot of what he displays is common for ND/autistic people, and someone having trauma doesn't necessarily rule that out. A large portion of autistic people have tons of trauma from their struggles in a world that does not allow them to be their natural self and punishes them for struggling to catch up to their peers, much like Zeke's childhood.
Zeke has a shit ton of trauma, but so does nearly every other character. I think the fact Zeke has these "weird" and "childish" behaviors compared to the other characters who have been through a lot as well means that something is fundamentally different about Zeke.
Basically, a lot of his behaviors are super familiar to me as an autistic person who has a shit ton of trauma. I want to clarify, though, that I think a lot of the other meanings to his behaviors can also be true along with being him ND. The neurodivergence would be underlying.