@bidaum No, it is actually an isekai, for no particular good reason.
As of page 6 of chapter 1, "I felt happy when I was reborn into this body... I am a prince! Even though I may be bedridden, compared to my previous life I can now live in luxury" he mentioned one or two things about earth at some other point too, I'd go looking but my internet is garbo right now. Regardless, the page 6 quote proves it to be an isekai, or at least to include reincarnation unrelated to the 1000 year coma.
In addition, the heroes are NOT (exclusively?) from that world, quite a few of them are earth heroes.
You'd be correct to say all his time spent in the hero realm had nothing to do with isekai, but you're wrong because for some reason there are isekai shenanigans that also exist despite not playing any particular role in the story. Easy mistake with how irrelevant the isekai elements are, but they still exist.