Analysis: why was it considered af blunder?

Why was this a blunder? Depending on Black's move, I see the chance of checkmate.
But is that the only reason?

Also I am impressed by the stockfish analysis.
It saw a forced checkmate on move 25, with 8 moves in advance. (Which I didn't)

Edited: Another forced checkmate lost on move 35, which I also missed, but maybe if I wasn't running low on time, I might have seen it?

It is the wrong knight and it allows the central counterstrike 6...d5. 6 Ndb5 keeps control over square d5, e.g. 6 Ndb5 d5 7 exd5 exd5 8 Nxd5 Nxd5 9 Qxd5 Qxd5 10 Nc7+.

6.Ncb5 does give the advantage away. After 6...d5 black's queen defends d6 and your knight on b5 doesn't have much to do now. However, putting a knight on b5 was the right idea! But only if it's the knight on d4. Why is that? Well now that we know the preferred defense for black is to play d5, we kept the on c3 to defend d5 AND opened up the Queen to defend d5 too. Now if 6.Ndb5 d5 7.exd5 Nxd5 8. Nxd5 exd5, black has a weak isolated pawn and 9. Bf4 (supporting Nc7+ with a fork) is hard to defend.

Think I see it now.
"Black's blunder gave White's knight a purpose.
But if black didn't blunder, white would have a knight hanging, and easily removed later by black's pawn. Costing white a move, loosing the opening initiative, and allowing black to strengthen their position.
Making the attack meaningless, and counterproductive."
Correct me if I am mistaken.

When you're a huge amount of material up, declared mates don't really matter too much.