I'd imagine that different people use the terms in different ways. Your therapists choice is probably theoretically motivated. To do with notions of responsibility and the like. Everyone has a degree of fragmentation. Some more than others, to be sure, but everyone has it to a degree. Some choose to focus on the differences. Your therapist is probably trying to focus on how they are continuous with you.

> when i switch into a different alter, 8 is 8. there is no me who is 32, there is no adult self present.

it may well seem that way to you, but the finding is that child alters perform on cognitive tests indistinguishably from how adults perform when they are trying to mimic the cognitive capacities of children rather than performing indistinguishably from children.

that isn't to say that you are pretending to be 8. it is just to say that you aren't in fact 8.

personality... persona... theory laden terms. if you are interested in what your therapist meant i'd ask them.