I don't get it: The lesson says "White cannot play 2.fxe4 due to 2...Rf2 checkmate". What's to stop 3 Kxf2?
Black has a bishop on b6 which supports f2, the rook cannot be taken. That said, the wording of the lesson isn't great:
With 1...Nxe4!, Black wins a pawn and went on to win the game. White cannot play 2.fxe4 due to 2...Rf2 checkmate. Black's knight sacrifice is effective, because the White king is trapped between Black's two rooks. White's pieces would remain passive after e.g. 2.Bb2 Nd6. Here we see an example of active long-range pieces vs. defensive short-range pieces.
IMO There is no sacrifice -- because the N cannot be taken, because of the checkmate he just pointed out. I suppose some folks call this a sham-sacrifice but as the N "can't" be taken (without major oversight from the opponent) -- why call this any kind of sacrifice?
I don't get it: The lesson says "White cannot play 2.fxe4 due to 2...Rf2 checkmate". What's to stop 3 Kxf2?