Yes, it’s only possible to specify branch names (more specifically, valid remote refs).
I’ve tried supplying a SHA-1 instead as branch and a build was created. But the supplied name is being passed to git clone --branch and the cloning thus failed.
Nothing prevents you from creating a topic branch at any commit though.
(See also https://stackoverflow.com/questions/14370157/git-fetch-a-specific-commit-by-hash for what valid input is in Git terms.)