Lol. Well, you can have the simple answer or the pedantic answer.
Simple - Yeah, sure. Its another world
or:
Pedantic - Well, what's your definition of another world?
In that case, it would differ from person to person. If we take world to mean a star or planet (as most isekais are not on Earth, but usually some mid-late medieval fantasy token world), then no, cause the first and the rest are not different worlds, just shattered reflections of the source, sub-dimensions if you will in a way. On the other hand, one could consider another world to simply be someplace that is vastly (or in many ways) different from your old world. In this case, yes, while reflections are basically sub-dimensions of Ethryis, so essentially the same planet just different, it's another world, hence you're in an Isekai. Take your pick I guess about how pedantic or specific you want to be cause the word Isekai is simply too generic and open to interpretation as to what you consider another world (be that a star, dimension, alternate universe, whatever). Remember, 異世界 literally just means "another world", combine the word "世界" meaning "world" with kanji for 異なる, which means "to differ; vary". And so it really is up to your interpretation of what "another world" is defined as. If we were to take the character meanings literally, that is in order, different/strangeness (異), society/world (世) and boundary (界), we can term to mean something like "different world/society boundary", in which case, Shadowbringers would comply cause the First is a different location, different place, and a different society and so would seem to match, but really, this is getting too into the weeds of it and so simply take the simple answer cause honestly it's most straightforward. It's a place not like your own, and substantially different from your own, so why not?