@WillLi
"isekai" is a very vague term in itself, it just mean "other world" (so in a way, 90% of fantasy story are isekai).
If we narrow isekai to what isekai reader think of, that another world is caracterized by one or multiple character having either:
[ul]
[li]Reincarnated (usually killed by truck kun)[/li]
[li]Transported (truck-kun transporting service)[/li]
[li]Received memory/information from someone/something that is not from this another world (I can only count two or three that I have this plot)
[/ul]
Now, the question is : does those memory necessarily need to be real or not?
if yes, this story isn't an isekai
if no, this story is an isekai
BUT! We could also argue "is isekai only necessary for the presentation of the story?" or "does the absence of isekai tag spoil a plot point of the story?"
I would argue myself that the tag shouldn't be removed.