But they rolled an insight check, which means they’re immediately locked out of Detect Thoughts. As you said, they either fail or don’t. If they fail they must immediately believe every word out of the persons mouth no matter what. So if they succeed then they must automatically know for sure, otherwise you’re giving a blatantly unfair advantage to the NPC. How is it fair that if they fail, they must believe, but if they succeed, they still must believe unless they happen to be a spell caster with access to Detect Thoughts? It’s not, so once the die hits the table they’re locked out. Which means the only possible use for Detect Thoughts if for the character to panic cast it at everyone they ever meet, because if the NPC opens their mouth and starts talking, the PC should have to make an insight check to see if they believe them without magic.

Personally I think a lot of it depends on circumstances. If the PC has absolutely no reason to distrust the NPC, then sure, it’s metagaming to cast Detect Thoughts. If the PC has some reason to doubt it, and the roll, as stated by the DM in OP’s example, DOES NOT RESULT IN THE PC BELIEVING THEM (explicitly stated by the DM at the time), then why exactly is the PC not allowed to cast a spell? That itself is metagaming in a different way.

Now if the DM has stated outright that the PC believes them, then yes, I’d say casting Detect Thoughts is metagaming unless the character is and has always been a paranoid character that casts it on everyone (which would get annoying fast). But that’s not what the DM said. I see the edit now saying the DM later stated that’s what he meant, which is fine. Just going off the original point made.