This is good and all. However, I would be wary of things like this. If what is mentioned did happen, wouldn't there by other sources, such as Meccan and Arabian sources. The Arabs were very good at documenting whatever took place, and they even documented many individuals they personally deemed as "false prophets." I am unable to find any mentioning of this story in any other non-Sikh source.
Also the transliteration and translation for the Arabic quote is wrong. It transliterates as "rabiy al'imam hadrat nanak, aklmuh 'ana fiha muslim" meaning that even in Imam Hazrat Nanak, one can see a Muslim. Things like this make me even more doubtful.