What makes Git different from any other software that does this?

Nothing, but git is a prominent example of software that treats history as immutable.

Isn't that sort of the point of git, though? To keep immutable records of progress being made on projects?

I think the article tries to raise the question of whether committer names need to be included in that immutable history or whether git could have done something else like using opaque committer IDs with a mutable ID-name mapping table.