Saying US military strikes on the 21st directly triggered a 'No' outcome for Polymarket on the 19th and 20th feels a little bit like suggesting a successful party on Saturday confirms no one turned up on Thursday or Friday; the absence itself is the proof.
It may be that the market resolved simply because no action happened on those earlier days, rather than waiting for a later event.
Things resolve based on what did or didn't happen at the specific time, not always needing a future event to validate a past non-occurrence.
For example, if you bet on whether it would rain on Monday, the answer isn't typically confirmed by sunshine on Wednesday.
The markets would have reflected the lack of military action on the 19th and 20th quite independently, perhaps even before the 21st's events.