How good is "that" good?

I genuinely think debating should not be about moderation but to explore the far stretches of linguisitical logic.

I have noticed that someone has said "K-Pop isn't even that good". By "that good", what are they even comparing it to, with nothing remotely related within the previous segments of conversation to make such a connection of even a little sense?

Colloquial speech confuses me.

"That good" means there are people who consider it very good.

So, person A considers K-Pop very good.

I say "K-Pop isn't even that good" as a response.

that was a conversation starter. what does that really mean.