I was never in the military, but know of an autistic boomer who is a military veteran.
You are correct on this point. The military gives autists the discipline and camaraderie they would otherwise be hard-pressed to effect in their own lives. This is not to speak of politics or foreign policy, but rather the institution of the military itself and martial tradition in general. What is true for NTs in this regard is so much more true for autists.
The main problem with the sperg in the OP,
as with the "socially awkward" kid who is getting bullied for being a simp, is that they lack male role models and are being exclusively socialized by women -- who do not and cannot know what steps they need to take to mature. It is certainly not being coddled like a little girl and told to be highly agreeable and get along with everybody!
Part of the problem, as well, is how autistic "special interests" are viewed in a patronizing way and even pathologized ("all-pervasive interests"). Clearly, the autist in the OP has a fledgling personality that is starting to constellate, but his sister is taking steps to undermine its development out of the Holy Liberal Imperative of harm avoidance ("save him from rejection and heartache"), and making him feel ashamed and suicidal instead because she does not appreciate that he's not a soy but rather quite based.