selective mutism is just as "real" as full mutism.
selective mutism is a severe anxiety disorder that can affect anyone. people with selective mutism find themselves physically unable to talk in certain situations or around certain people.
if you have a friend or child with selective mutism, and you wonder why they won't talk to you: know that it isn't because they dislike you, they just have an anxiety disorder that prevents them from speaking. however, forcing someone with SM to speak is harmful and abusive, and 99.9% of the time will not work.
instead, try asking them what their preferred method of communication is, and build a relationship with them, and you may even get to the point where they feel comfortable enough to be able to speak to you!
people with selective mutism are not "fussy", "stubborn" or just choosing not to speak. it can be an incredibly difficult disorder to live with. ableism does not make it any easier.
support those around you with SM. ask how you can contribute and help them. invest in learning the basics of your local sign language. treat people with respect; it makes a world of difference to us.