I don't know if this helps at all, but I think allistic people are able to pick up on each other's body language, social cues, and subtext because they instinct

I don't know if this helps at all, but I think allistic people are able to pick up on each other's body language, social cues, and subtext because they instinctively use the same ones. If an allistic person is tapping their foot while I'm talking to them, for example, I will be accurate more often than not if I assume they're growing bored with the conversation, because that's what would cause me to take the same action. If an autistic person is tapping their foot while I'm talking to them, there is every chance it has nothing to do with me, or that it might mean they're extremely interested in what I'm saying, and this is how they are coping with the excess energy of that excited interest. If I assume they're growing bored, I'll probably be wrong. I'm no more surprised by this than I am when monolingual people have no trouble understanding someone speaking their own language, but are completely incapable of understanding someone speaking a different language.