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I don't know what source he used,.Principal Satbir Singh was a Sikh author who wrote many books about sikh Guru's.

http://www.jsks.co.in/Satbir_Singh_Ji.htm

but on the other hand I don't find any reason to disbelief him on above.Guru hargobind ji must have written this kind of sermon anytime in his life as he was the first guru to martialise the sikhs .So at some point of life he may written sikhs to pick up the swords