Technically, you're right. Practically however, it does make sense to implicitly dereference the branch you're talking about to the commit it points to. Asking "is this commit on that branch?" is semantically equivalent to "is this commit reachable from the commit that branch points to?".

That said, it only makes sense to make such implicit mental shortcuts once you're already comfortable with git's data model. If you're not and you're still learning it, it's only going to confuse you.