Ok
The facemask was a judgement call. The rule states that you have to grab the mask and pull or turn the head. It clearly states that inadvertant fingers in the mask where the defender immediately lets go isn't a foul. Even a butterfly landing on the facemask turns the head a micron.
Regardless of the above, Goff was in the process of being tackled. The facemask didn't prevent him from getting a first down or scoring, unlike the penalty committed by the Rams at the end. That foul was committed precisely to prevent the receiver from catching the ball and getting at least a first down if not a TD.
Next, as you mentioned, the time factor made the later foul much more important. Had the facemask call been made, lots of things could have happened subsequently. They could have still kicked a FG. They could have scored at TD (which would change the playcalling on the subsequent Saints drive). They could have turned the ball over. Etc. This is in contrast to what would have very likely happened had the PI foul been called, which as you said would have meant the Saints sit on the ball, eat most of the clock, and kick the easy FG (which they did anyway).
In short, no one from the league is saying they missed a crucial facemask foul. But the league damn sure is admitting they blew the PI call. It was a much bigger and egregious foul at a much more crucial moment. I predict I will bring about some changes.