Quoting from OP:
> He's very concise, a bit rough around the edges and got a dry sense of humor. He told me he means well and has tried to be more verbose (slack is 1/2 of all our communication). He's also tried to tone down the dry. He really enjoys working with 2/4 folks who complained about him and wants to make it better.

Defining "reasonably normal" way of interaction is hard often to define, especially with limited understanding of all parties involved. People react differently based on their backgrounds and surroundings. Granted there are cases that can not be remedied and in those cases letting people go or isolating them might be the right thing to do.

> Folks in wheelchairs have totally different, physical, problems that are way easier to deal with from the POV of a company.

I agree that some disabilities can be easier for the company to deal with(ramps, parking, elevators, toilets etc. for wheelchair user). But even these things require to people to adapt("Why i lost my parking spot/office to a neckbeard in a wheelchair?" "Why we lost our end of the year bonus to get an ramp/elevator for one hire that can't walk like rest of us?").

Some requests are too demanding, but also some requesters are too demanding, but even then some requestees(/-ted?) are too demanding.

> Yet, this is exactly what is expected of healthy staff when people with, for example, Asperger join the team and I believe it is not fair and not right.

And yet this is what "healthy" AND "sick"(?) people must do. Asperger is a "spectrum" as a part of the "spectrum" of autism, which is within the "spectrum" of human behaivior. I belive it's not fair or right to expect everybody be like you or become like you. You have rights not to like someone, but they also have right to not like you.

IMHO it would be nice to see commenters to question more on the involved parties actions and motives. Also being involved in startup can be demanding and stressful job.

-Is the SDE's workload demanding to a degree that makes it difficult to him interact with people in verbose/"flowery"/"warm" way like those people expect?
-Is the SDE not working at full capasity or competently that makes friction? Or other employees for that matter.
-What kinda interactions they are having? Are they interacting in work related discussions or are they unrelated to task at hand?
-Is it the case of backlash to asking/requesting something (work/information) not authorized relating to work/products?
-Generational and other cultural factors in play? Are peoples expectations differing from each other in regards what they consider "reasonably normal"?
-Anklebiting? Case of slowly outing someone not like them.
-Is the SDE actually a problem case or other way around?
-Can some employees affect others views of the situation, ballooning the situation to over the treshold of complaints? Supportive complaints (under social pressure?) against the defendant?
-Politics! Do you necessarily want to talk politics(and other dividing issues) in an enviroment(like i see the USA from outside) where they can be career ending or atleast really harming to relationships?

More questions are needed to be asked and less kneejerk(?) actions taken. Based on information given in this post it's really hard to say anything definitive about the case. One might have experience where the kneejerk reaction of firing might have been the right thing and other have other kind of experiences of these situations.

It's disability not be able come to terms with other people disabilities. We need to be flexible, but not too flexible so we can still keep our heads up.

(Offtopic: How do you like my sorry aspergers behind vomiting questions which people should take into consideration? Are the questions above too insulting(not fair or right) to be taken seriously? Am I too insulting to be taken seriously just becouse I (want to) ask these questions or that I would like people to be better to each other? No hard feelings, just letting my diseased my drool over my keyboard.)