I wonder though - isn't accessibility just a constituent part of "good UI/UX design"? In other words, does it make sense to create a dichotomy between them?

Although I suppose you are calling out an explicit difference between "accessibility features" (like, say, a colourblind mode or specific assistance features) and something like a sudden spike in difficulty curve (which is arguably not specifically related to accessibility per se, but is more a broader game design/balancing question).