It's true the Polymarket resolution would be 'No' if no action occurred on the 19th and 20th, but the later military strikes on June 21st don't actually cause that prior 'No' outcome. The market for those earlier dates resolves based purely on whether an event happened within their specific window, not because something else occurred two days later.
That's a condition for the market's own rules, a sort of built-in fail-safe if nothing happens.
For example, if my flight to Spain was cancelled due to snow on Wednesday, it doesn't mean the flight I didn't take on Monday was cancelled by the Wednesday snow; the Monday flight just never took off for other reasons.