"Maybe you're hallucinating", let alone "let's say you're hallucinating", is a really weird take on someone thinking of a reasonable semantic distinction even if you disagree about its existence or relevance.

Perhaps you can say that to a friend as banter or in a tongue-in-cheek way, similarly to how you might say "I must be hallucinating" about yourself. But as an argument in a discussion with a stranger, it seems rather dismissive and inappropriate.

And it does reek a bit like something stolen from LLM terminology.