Scott Linehan has lost it
There is certainly some truth to what Linehan says; the fumble wasn't all on Kellen. That said, it does beg the question, why are 3rd stringers playing better for Rush than 2nd stringers are playing for Kellen? From my limited point of view, that could suggest the 3rd stringers have more confidence in their signal caller than the 2nd stringers have in their signal caller. It could also suggest that Rush is calling the correct protections at the line and Kellen isn't making good presnap adjustments to cover his backside.
So, when you balance that against the fact that Rush by far throws a prettier pass than does Kellen, I honestly don't know what is left to debate in terms of who is better? Does Kellen beyond a shadow of doubt do anything better than does Rush to justify being the primary backup to Dak. Once again, from my limited point of view, he does not. But Linehan's view is in no way limited; so if anyone could answer that question, it would be Linehan.
That said, you have to decide: Do you trust Linehan to know better? For my part, I trust Linehan...but I also acknowledge that there is a distinct possibility that Linehan is too inside the picture to see the frame; too close to both quarterbacks to make a fair decision. That aspect of the issue is considerably disconcerting.