When you edit a commit, it creates a new commit. They are immutable. You can still find the old commit via the reflog, until it gets eventually gc'd.

If I had to guess a reason they were downvoted (and I didn't downvote, to be clear), it's probably because people see stacked diffs as specifically solving "reviews clearly taking too long, too much content in there", and so it feels contradictory. Then again, as I said, I didn't downvote!