@Aruseus493 I'm pretty sure that we had this debate a while ago. Isekai is a genre because it is a set of themes that stories commonly use; any "person in a strange world" is classified as the isekai genre.
If we use your logic then yuri, yaoi, shoujo-ai, and shounen-ai, should be a theme of romance and not their own genres.
How about mecha into sci-fi?
How about tragedy into drama?
How about superhero into fantasy?
How about thriller and psychological into horror?
Plus, genres evolve over time. It's no surprise if we find their current usage to differ from the creator's original intentions. As long as the majority agrees with its current definition and usage, we can use the genre as is. In this case, the majority thinks that isekai should be a genre and not a theme.
@saaaad1 Bolding won't help so chill with the formats lol.
Also, if you're on mobile you can pinch to zoom in/out.
If you're on desktop, use ctrl (or cmnd) + scroll up/down to zoom in/out.