I think the argument is that if people tend to "just be rude" to women more often than to men, then that is evidence of sexism. Even if there isn't a conscious reason for the rudeness.
There clearly isn't any easy way to measure "aggregate rudeness" to be able to prove that women receive a higher quantity of "just being rude", but that doesn't necessarily mean it isn't there.
There clearly isn't any easy way to measure "aggregate rudeness" to be able to prove that women receive a higher quantity of "just being rude", but that doesn't necessarily mean it isn't there.