Why have people been saying BTC isn’t Bitcoin?

I’ve been seeing a growing number of posts that claim BTC is not Bitcoin. I have seen this thrown around from both sides, the maxis and the anti-crypto threads, but I’ve never seen an explanation. I’m assuming it’s more of a semantic argument than it is anything of substance. Is this simply referencing the genesis block or the Mt.gox fork? I would appreciate if someone could enlighten me as to where this idea comes from, as well as what significance this has on BTC and the over all crypto market if valid?