I think that the origins of the Sarbloh Granth are that when Dasam Granth was being compiled into a single volume, some of the works of Guru Gobind Singh Jee (that was not drowned) that was lost or forgotten found it's way into a granth which later came to be known as Sarbloh Granth. Over time a mythological story was created to explain it's origins like the one above. The sarbloh Granth still needs a lot of study. From what most of us have seen it does seem like authentic Bani of Guru Gobind Singh Jee, which is more the reason why we should further study it.