Yeah. You really need this caveat to avoid the loophole where you can compress anything to zero bytes by making the desired output a literal in your decompressor (which just ignores its input and prints the hard-coded literal).

This could be considered a real-world application of "hello world" programs: Winning compression prize contests by blatantly abusing the rules.

Well, yes. Except the source text is tiny, so unfortunately all the software is pretty big in comparison.

I love the Hutter Prize, and the various compression benchmark sites.

It's just pretty unconstrained without that kind of rule. And a decompressor that doesn't have to be fast can be tiny.