If we're asking what people mean when they use those descriptors, then you're correct.

However, literally speaking, in this context, immutable only means read-only, and atomic only means that updates are applied all-at-once or not at all (no weird in-between state if your update crashes halfway through).

The rest of the features (rollbacks, containerization, and immutable meaning full system image updates) are typically implied, but not explicitly part of the definition.

That makes sense, bazzite is referred to as atomic (that's what I meant in the above comment about atomic being more appropriate, forgot to add that context though lol) specifically instead of immutable. Bazzite updates like you said and you can always roll back, thank you for the explanation!