OP. You would have got short shrift from me. Sometimes I have 8yo DS2 in a pushchair Mac Major, when his asthma is too bad for him to walk when we are on the school run, and I have to get DS1 to school. I get on a bus. If anyone asked me if he had any SN's, I would tell them to fuck off and mind their own business, TBH.
I ALSO sometimes have 14yo DD in the mac major, if she has had a seizure and can't be left at home while I do the school run. Again, if you asked me whether she has SN, I would tell you to fuck off and mind your own business.
It is none of your's, or anyone else's business WHY my DC occasionally need to go on the bus in a buggy, and I wouldn't be telling ANYONE. I don't need to justify the fact that I use a buggy for them when they need it to ANYONE in RL. In fact, I might just be VERY sarcastic to you, as I am assuming this parent was.
WHY in the name of hell would you have a 6yo in a pushchair UNLESS YOU NEEDED TO. By the very definition, if you need to transport a 6yo in a pram, then there MUST be issues there - because an NT 6yo just WOULDN'T go in a pushchair!!!
In the situation you were in, I would have asked the lady with the EMPTY pram to fold down. If she ignored me, I would then tell her that if she didn't fold her pushchair, then I would fold it for her. And I bloody well would!!! (And HAVE done in the past.). Problem solved.
IMO, if you weren't willing to do that, then you should have just waited for the next flipping bus. Anyway - what would you have done had a wheelchair wanted to get on? Or a person with a Mac Major? Would YOU have folded? Or would you have got off? Those are the ONLY two acceptable options. Those spaces are for WHEELCHAIRS first and foremost. Using them for prams is a luxury that is only allowable IF no-one in a wheelchair wishes to board the bus, in which case, THEY take priority for that space even if there is a pushchair there already.