Interesting. Anyway, since they're going through the pain of changing the hash function, why not using the latest standard? SHA3 has been standardized for some time now, and using SHA256 isn't any easier than using SHA3-256.
They've already gone through the pain, deciding on it on 2018[0] (and functional & non-experimental 3 years ago per TFA). What's left is just changing the default (and some stragglers to complete support). Changing the function now would push back changing the default by a couple additional years until the new git version gets widespread deployment (incl. on LTS distros and whatnot).
[0]: https://github.com/git/git/commit/0ed8d8da374f648764758f1303...