I wanted to know this answer too.
I ended up going to their discord and found it.
You CAN get the hisuian starters - at least if you start over or have one of their non-evolved forms to evolve(?). SPOILER INC:
Hello!
I went looking but I could not find confirmation so I thought I'd just post an ask about it.
Are Hisuian pokemon in the newest Desolation update? I am restarting my game from scratch to enjoy all the new content, and