Double Commander, which is done in Free Pascal/Lazarus, doesn't look or work too integrated on OSX—though I don't know if that's possible by spending more effort. It's using Qt for the looks, as the Cocoa variant works even worse—and that's for some reason inconsistent, compared to other and perhaps simpler Qt apps that aren't made with Pascal.

A file manager has to plug into multiple native features, e.g. the list of app associations—and DC does that sorta half-assedly.