If you call a coverage that has Kyle Van Noy turning and running downfield with Kenny Stills, then that's on you as a playcaller because it's a terrible matchup that he'll lose 95 times out of 100.
After looking back over the play, I think I can at least plausibly guess where the coverage broke down. There were two corners lined up to the left on two receivers. One receiver stopped short, the other went deep, and both corners broke on the short route. That left an open receiver deep, and McCourty committed to helping on that receiver. Which meant that there was nobody over the top to help Van Noy.
At the time, my guess was that one of the corners on the left blew his assignment, and that had it been done correctly McCourty wouldn't have needed to commit and would have been in position to help on Stills. If what Belichick said here is true, then maybe they were trying to bait Moore into chucking it deep down the sideline where McCourty could make a play on the ball, and it backfired when Stills ended up in the exact space that McCourty vacated. I doubt that, since it's unlikely that Moore would've made a pre-snap read based on the coverage the Pats were showing that would suggest he should make that throw. More likely, maybe they were trying to bait Moore into making a pre-snap read that would indicate throwing to the short route, which would become an extremely high-risk throw once that guy turned out to be bracketed. In any case, it didn't work out, because Moore wasn't baited into anything and instead he had a wide open WR streaking down the middle of the field covered by nobody who has any business trying to cover him.
I dunno, it was an odd play. I'm also mildly skeptical and think that Belichick may be covering for someone here, but in this case I think it's more likely that he's covering for whichever corner may have been responsible for the downfield receiver that McCourty had to pick up. One thing I am pretty convinced of is that Van Noy isn't at fault here, because I don't believe Belichick or Patricia would ever ask him to cover Stills downfield man to man like that. They're all about finding winnable matchups for their guys, and Van Noy on Stills is pretty much the opposite of that.