@fourthsheep I think that explanation is a bit of a stretch, since even in the many worlds interpretation spacetime moves "forward" at the same rate, so moving to an "alternate world/parallel universe" would have to put him in a world very near the same time as his own (much like the activation of Okabe Rintarou's Reading Steiner in Steins;Gate). Such a movement shouldn't violate causality (i.e., no faster-than-light travel), whereas moving back as much as 13 years would require performing something akin to a time leap and violating causality.
Besides, isekai generally involves the protag moving to a different world than his own and leveraging some unique knowledge or skills not native to the new world. Given that Shikkaku Mon no Saikyou Kenja
https://mangadex.org/title/21901/sh...sara-ni-tsuyokunaru-tame-ni-tensei-shimashita doesn't have an Isekai tag and features an OP protag who travels
forward in time within the same world and leverages his "cheat"-like knowledge, I think it would make sense to remove the Isekai tag here as well.
TL;DR: Isekai carries the connotation of the protag moving to a
different world than his/her own, which this protag does not. Given that other similar stories don't carry the Isekai tag, this one shouldn't either.
