I'm not certain that I agree with this because a URL makes no claims about idempotency or side-effects or many other behaviors that we take for granted when building systems. While it is possible to construct such a system, URLs do not guarantee this.

I think the fundamental issue here is that semantics matter and URLs in isolation don't make strong enough guarantees about them.

I'm all for elegant URL design but they're just one part of the puzzle.

Yes It does. HTTP PUT is idempotent.