Well, first, I'm talking not just about the subway, but all rail lines, which is a higher bar.
Setting that aside, what I meant was what you've inferred -- there's typically one elevator path that will get you out of an "accessible" station. Yes, that (usually) requires one elevator per platform to work at all, but it still leaves most exits inaccessible in the average case.
I started this subthread only by saying that Tokyo is not particularly wheelchair friendly. If you use one, I don't know how you could possibly dispute this. It's a land of staircases, and I've been repeatedly foiled just trying to move heavy luggage.
I grant you that in Japan they will usually bend over backwards to make something work, but I really feel for disabled folks here.