Sure, my point was that a single instance outside of other context means that you cannot necessarily discern a pattern of behavior upon which to base your conclusion into which kind of asshole he is being. You could be innacurate in assuming he is sexist as well as assuming he isn't. If complete accuracy is required, then you would need to not make a conclusion at all and let the comment slide without feedback until you have more data. I'm saying that it is more important to call them out than to worry about the exact accuracy, to not let the comment slide, to make sure they know that, in some way, it was inappropriate.

One's experience may lead one to make some assumptions that are incorrect in this context, but I don't feel like that is the important part that you should critique. Either she says nothing, calls him a sexist, or calls him out but doesnt point out the sexism even if there is unconfirmed sexism involved. I'm saying either of the latter is reasonable under the circumstances.