"15 words" is a secretly a verb rather than a noun.

As the thread creator you are entitled to specify the way you want the phrase to be used and what sort of replies you want. That said, it seems that the norms that you are attempting to create and enforce for this '15 words' activity don't belong on this site. It seems to amount to provoking and enforcing all the worst of the failures of critical thought that constantly crop up in the "Rationality" Quotes threads. Given as a premise that I hold that belief you could infer that my voting policy must be to downvote:

• Any thread or comment requesting the 'action' "15 words" be performed.
• Any attempt to criticise, suppress or dismiss clarifications, elaborations and analysis that crop up in response to quotes.
• Any comment, regardless of overall merit, for which a minor clarification is necessary but would be prohibited or discouraged. Note that this applies to the ancestral quote by Pearl which I had previously upvoted. In a context of enforced uncriticality any deviation from accuracy becomes a critical failure.

I'm sad to some people engaging solely in that and missing an opportunity to try out the exercise instead.

That isn't what you saw. You saw people engaging in that in addition to engaging with the the exercise. They lost no opportunity, you merely couldn't tolerate the critical engagement that is an integral part of discussion on a rationalist forum.