The main difference to your examples is that an “immutable OS” is in fact mutable, while none of your examples describe themselves with an adjective that is contradicting with their function/inner workings.

Flatpak is a pretty good name, because it makes software flat in the sense that it avoids having a (tall) dependency tree.

I print from my phone just fine

Ah yes, the immutable OS, except for all of the various mutable parts.

We should totally not call it anything less confusing.

How could you install anything or change any setting if it “doesn’t change” ?

How could you install anything or change any setting if it was truly immutable?

Immutable OS makes sense in certain scenarios, but not in home computing.

I second this.

I’ve learned about it at work and used it privately.

I use Dokploy and I think it fills exatly the same role.