Well, it is still not an isekai, it's a regression. Semantics, my friend. 🥸
Of course, I'm just bickering, I'm not really seriously criticizing. But for those curious:
"Isekai" (異世界) means "[transported to] the other world". It is also called "transmigration".
This series however is a "Regression" since the MC regressed through time. It can also be called "Return", but "return" is a more general term for any MC returning--whether through time and/or space. When a person who was isekai'd or thrown into a different dimension comes back, it's also called a "Return". For the sake of distinction, most people exclusively specify "Regression" for MC who went back in time, while "Returnee" refers to MCs who returned to a place they were in or a position that they once had.
Lastly we have "reincarnation" where the MC did die/disappear but later on inhabit a different body, often in the same world. Now, reincarnation is quite broad as it focuses on the cause of transmigration/regression--which, for reincarnation is, of course, death.
An isekai can be a reincarnation if the MC died before being transmigrated to a new world, but for clarity, it's simply called isekai'd--especially if Truck-kun delivered them.
Similarly, a "Returnee" can also be a "reincarnator" if they died in the world they returned FROM and instead inhabited a new body in the world they returned TO.
Regressions on the other hand, often trigger on death, and since it's about turning back time, the MC isn't really considered to have reincarnated even if they did inhabit a new body, as in a body that wasn't theirs before they died--such cases are only considered "Regression" and/or "Returnee", depending on what aspect of the return the author wants to highlight.
All of these are just semantics, by the way. Tropes evolve after all.