I've seen these type of desperate decoy or blocking moves given best move by the engine when the situation is heading towards mate-in-x moves. In this case, the Knight is offering to be captured by the Bishop to temporarily stop the Bishop moving onto the same diagonal as the White Queen and making a mating threat.
How is this not a blunder?

To determine if an action or decision is a blunder, consider the context, intent, and consequences. Assess if there were alternative options, and if due diligence was exercised. Mistakes happen, but understanding and learning from them is crucial. Without specifics, it's challenging to judge conclusively.
ChatGPT type response
very bad position so not a blunder