Has the thing about ascending with dreamselves on Questbeds and without on sacrificial slabs been confirmed? Or is it just a theory based on the ascensions we've seen so far? And where did the name "sacrificial slab" come from?

Calliope uses the term here and mentions that since the Alpha kids have no more dream selves that dying on their Quest Beds would do them no good and that they need to use their Sacrificial Slabs at the center of Derse and Prospit’s moons.