isekai is a sudden event so it doesn't apply to their situation but I get what your theory is. And wasn't it already mentioned that is like, levels?? An example is the mom pointing to the kergo guard and saying he's close to being granted eternal life or smth
Isekai is a Japanese term that literally translates to "another world" or "otherworld." So it's correct to use it here cuz they are literally in another world regardless how they get there.
Also they still refer those levels* to be in heaven. So this folks specifically the mecha tribe all thinks they are already in heaven.
They call this place heaven, but I think they actually just got isekai’d to another world. There might be another realm there equivalent to heaven & where those angels are from. The place we’ve seen so far is probably just the ground level where three different clans reside.