The problem with Landlock, AFAIU, is that it's inherited across exec. OpenBSD's pledge and unveil were deliberately designed to reset across exec by default precisely because the "more secure" behavior makes it difficult or impossible to add to alot of existing code that needs to invoke other programs. It could be done in theory--e.g. by forking a helper process prior to locking down--but that requires a significant refactor all its own, so what ends up happening is that people just abstain from using seccomp or Landlock. Whereas all OpenBSD core utils were quickly modified to make use of pledge and unveil to some extent, and then over time improved to tighten things further; they were designed to permit and promote this sort of easy, incremental deployment.
I don't know the extent to which Landlock or even unveil would have helped here; maybe they would have only prevented the hook from running during the clone, but not subsequently when it's expected trusted hooks to run. But I'd bet adding unveil support to Git would be an easier task, notwithstanding (or even because of) the way Git invokes subprocesses.