@jasty Technically, this manga classified under isekai, which is when the characters move between worlds; a very broad definition. However I do not know if this is what the original Japanese definition meant. Because in this sense, literally Steins;Gate would be an isekai (just think about that for a second...).
I don't mind the tag being here, as it's not wrong. But personally I don't think this counts as isekai. Isekai stories usually focus on the self-insert and RPG/fantasy adventure. Stories like these focus on the sci-fi and ways to get back or travel to more universes. I don't think people will mind the isekai tag removed for this one either.
Some fun fact: Not a common term, but these kind of people are usually called "sliders". From the Suzumiya Haruhi Wikia:
The sliders (異世界人, isekaijin, people from other worlds) in the Haruhi Suzumiya franchise are, supposedly, humans who jump through different dimensions.
The term is used outside of the Haruhi franchise, but it is a very niche term. Isekai stories are not about the technicality of going to different universes, or finding out
how it works. Which is why I think the term isn't popular since not many stories are like this manga vs. the amount of generic isekai.