This feels like declaring squares aren't rectangles because you can make rectangles that aren't a square.
Commits link to their parent(s) until the initial commit. You can visualize a “branch” as a series of commits, but the same thing can exist without calling it a branch:
A -> B -> C —> Initial
Is that a branch? No. How about: /refs/heads/main = A
That’s a branch, and it looks exactly the same as a tag: /refs/tags/v1 = A
Neither a branch or a tag is “a series of commits” even if you think it does.You can visualize a "square" as a rectangle with equal sides, but the same thing can exist without calling it a square.