"What does this claim even mean? I don't think that anybody thinks including Apache code in their GPL project would retroactively relicense the code of the Apache-licensed project that they probably had nothing to do with, written by someone they may have ever met. Is this what you're confirming?"

The comment i replied to literally said that - that you can always relicense things less permissively. Which is wrong. Sure, maybe they meant it in the colloquial sense, but you simply can't have meaningful discussions about any of this without being precise.

"That's not really how code works. The old code is mixed with the new code, and the combination (I've always thought) is going to have all of the restrictions of both (all) licenses involved."

You certainly realize how arrogant it sounds to tell someone "that's not how code works"?

I'm quite aware of how code works. No, it will not have the restrictions of all the licenses involved, just because you mix it, because it can't. It may be hard to figure out what the restrictions are, but just because you mixed it all together doesn't change a darn thing. You still have no meaningful separate copyright in the mix, and the licensing is controlled by the pieces.

Read again what you claimed is obvious above, and you will see why.

The only restrictions come into place to the degree you create derivatives of existing works or code. What degree that happens varies widely. What is protectable there, varies widely.

It is totally irrelevant how much chaos you create through mixing - it does not change licensing, just makes it harder to figure it out.

Court cases that involve open source with lots of differently-licensed code often spend amazing amounts of time trying to actually separate the pieces and figure out what rights people have to them.