In this case it isn't a "in every way better second system designed from the ground up to include a kitchen sink", it's just an API-compatible rewrite in a different language.

Yes, but it is normal for sayings like that to have contradictory versions, so that one is available in any situation.

For example:

"Too many cooks spoil the broth" vs. "Many hands make light work"

"Look before you leap" vs. "He who hesitates is lost"

Or for a single daying to have antithetical interpretations:

'It's just a few bad apples" vs "one bad apple spoils the barrel"

Or "blood is thicker than water" can mean you prioritise family.

But the full saying is actually "blood of the covenant is thicker than water of the womb" and means opposite

I've never heard that "full saying" and Wikipedia says that people who claim this offer no citations for it. One of them is a 9/11 Conspiracy Theorist, the other listed is from one of the crazier than average US religious sects.

It's not a second system but instead a rewrite, since they are not making new architectural decisions.

In this case, aren't they constrained by a gigantic set of test cases? That can even be run automatically?

I am not sure if I can associate it with a rewrite. Can you elaborate a bit?