This is effectively required when open sourcing something that was previously internal. I've done it at a company. There could be company internal things that leak in old revisions of code and commit messages. Even if the latest commit on master is clean, it is really hard to know that every revisions, throughout the whole history is clean.

Good point. In such a case, I think it would still be nice if at least trying to split the total change into a few separate commits, that builds upon each other. That way the commit log could be a good start to look at for someone who wants to understand the code base.

With the first commit being the specific commit from the PostgreSQL repo which they base their work on, i.e. that they forked from.

They are following the Gaussian approach. Show only the final result!