What makes an isekai is the whole “dying and reincarnating” thing,
It's not. The "dying and reincarnating" thing only became a trend in isekai stories relatively recently. Most classic isekai stories involved traveling to another world through a portal or the like and ended with returning home.
Time travel can also be a form of isekai. Arguably one of the progenitors of the genre is "A Connecticut Yankee in King Arthur's Court" by Mark Twain (especially the trope of bringing modern technology and culture to the other world), and there are other examples like "Buck Rogers in the 25th Century", "Futurama" and "Planet of the Apes".