With utmost respect to Sikhism. I request you to not use such story for Shanidev. He is a Guru himself for many. There is no need to fear or believe in him. He is a path in himself. Don't disrespect something and say it's nonsense. People worship him because it worked for them. Many paths to the same destination maybe. It's about the devotion anyways not the Iron, mustard oil or Urad Daal. The culture says Mata Pita Guru Devta.