But the New Republic is what's around and they don't even know there's anything more than Imperial remnants left.
No way the Empire can still remotely be occupying Mandalore if that's the case. Especially when Gideon was on the run and arrested and Thrawn is somewhere in the Uncharted Regions with Ezra. Would they introduce another Imperial Officer running things?
Plus, Din was flying around Mandalore and it's moons undistubed and Bo was even surprised to even see TIE Interceptors in the first place. I'm not saying you're wrong, I'm just using the evidence with what the show has given us.
But like
@Garlador says, we're probably giving too much attention to something that they'll easily solve so it is what it is. Shutting off my brain now and will just blindly consume these final two episodes.