The solution to this puzzle is amazing! 

"If out in the open"

That does mess up the stairstep gambit, yeah, since the knight is now blocking the bishop from controlling b1. I ran it through some chess engines and after BxN white still wins, but it's messier. If the black king continues up, white wins in about 8 moves, if the king moves back down and to the right it's mate in 19 moves, but neither is an immediately obvious sure thing. Both involve using the bishop on black to keep the pawns controlled while the bishop on white and the king shuffle the black king to a good place for the other bishop to dive in and finish things off. I think that this video didn't cover this because after the knight sacrifices, the general rule is that the bishop controlling the pawn promotion square leads to a certain win with good play as long as the black king isn't right on top of the pawns to protect them anymore.

50-move rule left the chat

Shut up anonym5160

In the most positions it is not a draw by 50-move rule.

Anonym5160 shut up anyway

In most positions 2 knights vs pawn is rather a draw than 2 bishops vs knight.

After white captures the knight with check, Black goes Kb2 to escape the corner. Than white Bd4+ pushes the king out and ends the threat of promotion. The Bishop could plant himself in a1. This allows plenty of time for the white king to journey across the board to clean up both pawns. Two bishops against a king is a winning endgame, but not as pretty as the mainline.

I'm so glad you mention this! That is actually one of the most watched videos on my channel and the puzzle is from the same composer. A link to that video here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-dZ1rXNfeaRQ.html

6:48 Nc3 Bxc3# 10:05 it is white turn, black just played ... Kb1

At 6:48 white would take the knight with bishop resulting in checkmate. At 10:05 black can’t promote since the king is in check and must move to the only open square at A1 which blocks the promotion.

after Nc3 then Bxc3 wins. at 10.05 black does not have the opportunity to promote

Wym bro black doesnt have a move to promote

Because his king is in check. Bishop on d3 is checking the king on b1.