As far as I know, commits that were once ancestors of a branch can sometimes no longer be ancestors of that branch. If that's not a mutation, then the word is not as useful as I thought.
Branches are just pointers into a graph of commits. The graph cannot be mutated (it can be pruned by garbage collection but that does never happen related to an oh shit event, it takes weeks even in a heavily modified repo). The pointers can be modified to point anywhere else in the graph, which makes things look mutated without being mutated. Using reflog you can get your back to where your pointers pointed to at any earlier point of time.
I'd say it's a pretty standard use of the word. Branches are analogous to pointers. If a program changes where the pointer points, it has not mutated any values, besides the pointer itself.
