That seems like a perfectly valid primary source to me.
To quote Mathilde when she dropped Queekish off-
"My sources are primary."
And honestly I think as long as we didn't touch human gods there wouldn't be much blowback that we couldn't run away from. I think of it like the dwarven equivalent of the colleges discovering we read the Liber Mortis.
But honestly, there is apparently a LOT more unknowns than knowns regarding the gods, and I think some epistemic modesty from the thread at large is in order. We don't know Hashut *isn't* a dwarf. We don't know if the ancestor gods were a rebellion, a military expedition, or refugees from a hold already fallen. We don't know what happened to the first generation (s?) of human gods, or what the difference is between a minor god and a major one.
I've tried to keep my phrasing as speculative, and it's a bit annoying that it's largely been met with opinion stated as flat fact. It's a style of Internet arguing that I kinda hate because the only thing it does is escalate.
Seeing this with the Dwarf diet thing too. We don't actually know what the root cause of the 'malaise', lowered sexual drive, and early deaths are, so statements about it being obviously psychological rather than physical are just priveldging a grey wizard's viewpoint (trained in psychology) over a jade wizard's (trained in husbandry and nutrition). I think the old saying about looking for keys under a streetlight is applicable.
So- I *think* Cython is on to something, and it's thesis of rebellion fits well with what we know of Grimmnir killing the dragon, and the utter lack of any information of times prior. (Almost like it was deliberately erased.) I also strongly think that the differences between the domains of the gods worshipped by the dwarves and those worshipped by other species imply an earlier dwarven pantheon.
I think, to a lesser extent, that the idea of an abandoned pantheon coming back as horrific parodies of themselves fits with the Warhammer aesthetic, and gives us answers to two open questions- "why is Hashut different?" and "what is Gor-dum?" with a single move, making it somewhat simpler than other possibilities.
I'm open to the idea that there are non-dwarfs being worshipped as ancestors, because worship done properly to a dwarf is worship of ancestors, and so the idea of a separate non-ancestor god might get blurred away by cultural pressure.
They've been in Araby centuries ago though and I at least thought they've spent at least a few decades looking into non-Elven gods.
I guess I just expected them to be more focused and driven regarding their hobby than they actually are. In a way it makes sense though. Hysh can have a bend that is more philosophical than empirical.
I think you are greatly underestimating the difficulty a dragon would have in just talking to non-dragons. The likelihood is that they either immediately attack, or more likely, flee in terror. How would cython have gone about buying books or holding conversations?