Hmmm, when people refer to an immutable distro, they’re talking about a system that has a read only base system, atomic updates (with rollback), and primarily (or exclusively) containerized applications.
That describes ublue.
When people refer to a Linux distro, or if someone looks at the Wikipedia entry, ublue definitely qualifies as a distro.
Bazzite, Aurora, and Bluefin are immutable Linux distros. The fact that they are themselves containers, and use bootc, doesn’t change that.
I’d much rather recommend people use Bluefin (which I do, quite regularly) without having to tell them that it’s not a distribution, and also that it’s not immutable, even though it fits EXACTLY what they are expecting with those two words.