It's a 17 minute video.. that's quite the round up. Just saying because for me the difference from 15 minutes to 30 minutes tends to go from: "yeah I'll check this out" to "boy this is an investment".
Interesting, I'd assumed it was going to be a pawn promotion to something esoteric like a knight that didn't create a check, but that's a couple layers deeper.
On a scale of billions of games, your situation would be be fairly common. The author of the video got to the conclusion by stacking multiple insanely rare occurrences - the player would need to underpromote to a bishop (the rarest promotion) and then capture and checkmate while these bishops are placed in a specific configuration. The author get into weird territory fast when fishing for the strangest game out of billions.
It's a 30 minute video. If you care that much about the topic, enjoy. If you just wondered what the move was... you're welcome.