They were mentioned because a file they are the code owner of was modified in the PR.

The modifications came from another branch which you accidentally(?) merged into yours. The problem is that those commits weren't in master yet, so GH considers them to be part of the changeset of your branch. If they were in master already, GH would only consider the merge commit itself part of the change set and it does not contain any changes itself (unless you resolved a conflict).

If you had rebased atop of the other branch, you would have still had the commits of the other branch in your changeset; it'd be as if you tried to merge the other branch into master + your changes.