> I hate it when people say that to dismiss things.

I always avoid ever saying anything like "that's [just] semantics", and I try to discourage others to, too, but in many of the situations where it shows up (including this one), there's nothing wrong with the complaint itself. When you hear someone swipe, "I'm not going to argue semantics with you", it generally carries an implicit accusation of an attempted change in subject. Consider:

Sara: "Nathan ate the clementines I bought from Oliver this morning!"

Oliver, Nate's friend: "I didn't sell her any clementines. The only fruit I've even touched today are these. They're not clementines. They're small mandarin oranges."

Does it matter that Sara can't distinguish clementines from ordinary mandarins? If there's a law that punishes you differently depending on whether you stole true clementines or not, then sure, what they actually are is materially relevant. Absent that, though, does Sara care what the definition is right now? No, and neither should anyone else. To even bring it up just distracts from Sara's actual complaint.

That is to say: if the argument starts as a semantic one, then semantic quibbles are relevant. ("The law says this about subcategory Y. Does what's at issue satisfy the definition of Y, or is it merely Z?") If it's not, though, then it's appropriate to call someone out on DOSing the conversation with a semantic rabbit hole (instead of being able to concede, "You say 'photocopier', while I say 'Xerox'", and then moving the fuck on).