The difference is Luffy has no idea what Nika means, while Zoro is very familiar with CoC. It's not like it's a mysterious ability he's never heard of.
He doesn't have to know a thing about Nika to be curious why he's being called that.
Or the fact G5 makes it extremely he's not a Paramecia DF user.
The fact no one questioned his new appearance is absurd, but it's consistent with Oda.
Do we know for a fact sensing in CoC is an identical sensation/mechanism to feeling it? Regardless, it's stupid Zoro didn't immediately know he awakened CoC. I also find it stupid he didn't learn any advanced Armament techniques in Wano of all places. He should've learned CoA Emission at the very least.