I agree with you for this particular case but you are mistaken if you think that sarcastic and ist are mutually exclusive. Sarcastic comments of a ist form can (and nontrivially often do): mask sincere ism; be a more socially-acceptable-looking way of expressing actual ism; encourage ism, especially of a casual degree; exhibit societal/subconscious/not-actively-malicious ism; or create a generally hostile environment for people of an oppressed class of *.

So, yeah, probably an overreaction for this particular occasion, but it's not so easy as writing it off as "sarcasm".