Actually, it looks like Skeld and Volkor are from the base game, but like everything else, they can be used in the Ancient Chronicles as well.
And just to be sure, I checked the new heroes and they all say
Additional hero for Ancient Chronicles that can also be played with Immortal Souls.
although that was submitted by a BGG user and not the publisher. I do recall the KS saying that all of the content could be mixed and matched, though.