Same thing, both words describe the same thing.
EDIT: I have learned not only from the dude below me, but also more reading online that indeed atomic and immutable are not exactly the same thing. Related but different. Tho my comment stands I still like the atomic nature of Bazzite, but I also LOVE the immutable part of it.
TLDR: Immutable means the core OS is read-only while it’s running, and atomic means updates swap in a whole new system all at once, so you can always roll back.