a 600 elo player checkmated me with Knight and Bishop

If he only had a knight and bishop on the board, and he had to actually set it up, it's EXTREMELY unlikely they could've done it it without cheating. Maybe if it was a daily game, and they watched a tutorial on how to do it, they could pull it off. That being said, If there was little or no setting up needed (e.g. your king was already in the correct corner, and the piece were already basically set up for him), it's possible he accidentally pulled it off.
When I read a lesson on it, I can do the checkmate. But I soon forget it. I find very difficult to get the king from the wrong corner to the right on. It is easy once the king is in a 3x4 area near the right corner.
600 players pulling off that mate seams very unlikely. I watched them make at least 10 blunders in a game.

That’s hilarious!
I have only one opportunity in the thousands of games played, and I could not do the mate myself; took the draw after a dozen moves.
I don’t think it’s worth to learn that mate considering how rare it is. You’re more likely to catch a Leprechaun.

Even i can't do that lol
Don't feel bad. Not long ago one of the strongest women players in the world couldn't do it either! And I once played against a strong master that was not able to do it! And I would bet that there's not a 600 rated player on this Earth that could do it! (Unless of course they cheat.)

That’s hilarious!
I have only one opportunity in the thousands of games played, and I could not do the mate myself; took the draw after a dozen moves.
I don’t think it’s worth to learn that mate considering how rare it is. You’re more likely to catch a Leprechaun.
I agree 100% with Chuck! It is not worth the effort learning how to mate with Bishop and Knight. And if it ever does begin to look like you're coming down to just a Bishop and Knight, then instead protect your last couple of Pawns (or especially your LAST Pawn) at all costs!

Even i can't do that lol
Don't feel bad. Not long ago one of the strongest women players in the world couldn't do it either! And I once played against a strong master that was not able to do it! And I would bet that there's not a 600 rated player on this Earth that could do it! (Unless of
Oh how wrong you're! It's a piece of cake.
I challenge you to set up a position (any that you can't capture my knight or bishop, of course) to prove you you are wrong. I'm not even in top 100 in a tiny Croatia.

I agree 100% with Chuck! It is not worth the effort learning how to mate with Bishop and Knight. And if it ever does begin to look like you're coming down to just a Bishop and Knight, then instead protect your last couple of Pawns (or especially your LAST Pawn) at all costs!
The purpose of learning the process of mating with a knight and bishop is not about learning the mate itself, but rather how to coordinate minor pieces and how to restrain your opponent's pieces.
A few years ago, I was at a local tournament when an 1800 and a 1500 reached the BNKvK endgame (the 1800 was on the losing end). The 1800 walked over to the tournament director and tried to claim a draw by insufficient material and was shocked when the TD told him that was not a valid claim with the given material. His opponent spent the next 50 moves trying to figure it out (and got close 3 times, but missed the key point about restricting the enemy king). It ended up being a draw by the 50-move rule.
Now, is it possible someone rated ~700 could know the process and be able to execute it in a rapid or blitz game? Sure, but it is highly unlikely!

Did you force him to execute perfect technique? Or did you voluntarily more your King to one of the wrong two corners and make it easy for him?
It's difficult to do against the best defense, poor play by the defender can make it much easier.