I had a theory that I think is crazy enough to possibly be real.
Vasher/Zahel is the one writing it
the writing reminds me a lot of the way he talks when he's explaining deep stuff to Kal and Vivenna,
we know he's a scholar.
as for hearing the Wind as a child, do we know how old he was when he came to Roshar and got inspired to make Nightblood, it's possible he was a child and ended up getting saved by a shard bearer and that's what inspired him later to make our favorite murder sword.
Another possible idea is that his parents had been immigrants from this and he was raised completely onf Roshar before he eventually went to Nalthis and became Returned