A warhammer planet appearing in Douluo Dalu world, is an isekai. So yes. Isekai is basically quite a "broad" theme as its simply "another world". Though for anime community and/or tagging it is more in the sense of "hey i reincarnated into --- and my vibes are jp style"
BUT. Now i think about your question. It maybe not, as it depend on a certain VERY particular detail. The "POV" Is the POV of narrator/novel/MC an aborigen ? OR from one of this people on the island nation that appeared ? Depending that, it may or may not be isekai. Just like MC=aborigen=fantasy and not isekai.
It's from a grimdark parallel earth. The main character isn't even in the first half of the book, and is not the only pov, but is the nexus upon which the story moves
Well, it is an isekai for the inhabitants of the island. It is an invasion from the perspective of Earf.