Ah, it wasn’t clear to me which one you were referring to. In his case, his feelings aren’t a valid part of the conversation. His problem is his inability to separate two completely unrelated words, not the word itself. Assuming he actually exists and his problem with cis really is just that he can’t separate it from a completely unrelated word, that’s still not relevant to the larger conversation, it’s at best a pointless aside, at worst a distracting tangent for intellectually dishonest anti-trans people to use to derail the conversation.
And the fact that every time I’ve seen that brought up (other than possibly this time), it’s been to derail the real conversation is a big part of why I distrust that the guy really exists; and why can’t believe, if he does exist, that he really just can’t deal with two words sounding similar but being unrelated.