Some day in the distant future someone will only have a tarball of the source of the last version that worked; if they're lucky.
Save that poor sod; have your comments live in your code, not in an SCM system for your code.
GPL has this idea in it: trying to define what exactly should go into that tarball. The question seems meaningful and important even for proprietary code. If some authors assume that anybody modifying their code should be able to look at git commits, that probably simply means that "the preferred form of the work for making modifications to it" is a git repo.
Yes, text is better for convenience and for being future-proof. It's like leaving a description of the bug I'm fixing, instead of just a JIRA ticket identifier.