It's not like astral projection is some weird thing Billie's never heard of. It's still a weird thing, but she has heard of it! At least enough to know it exists.

"I'm pretty sure getting into the shadow realm takes a completely different trick than whatever you've been practicing," Billie can't help but point out. "All other weirdness aside, you're kind of, you know, alive. Are you sure you want to take chances like that? 'cause I've already told Strange, the only way I know how to deal with a possession is killing the host."

Admittedly, other people may have a better idea, Shura included.