At first, you just think Poe is being snarky and posturing because “HE’S THE COOLEST PILOT IN THE GALAXY AND IT’S HOLLYWOOD MOVIE RULES! YOU CAN’T BACK DOWN FROM THE VILLAIN!”
You watch their scenes again, though: Poe is clearly bemused by Kylo and what he’s become. He’s seen behind that mask.
He’s still scared, obviously. But he seems to know, for a fact, he’s not quite dealing with a monster here. Because he knew him before.