I find this hard to believe, as I know a lot of Sikh men who never cut their hair their entire life, and their hair is super super soft and fine. Too soft by usual standards, and much softer than any woman's, even on their face. This is not genetic IMO, as two men in the same family can have radically different hair quality where one is traditional and never cuts and the other cuts and shaves like a non-Sikh.
Another thing... you can often tell which baby has had their hair cut before and which hasn't when they are very small based based on the strength of the hair.
In many cultures a baby's head is shaven to ensure strong hair growth and it seems to work. Again IMO, not science, but keen observation which is sometimes more reliable.