Review their track on page one of this thread.
Looks to me like they hit Redwood Falls, the last fix on the normal KSAN-KMSP flight plan route and the first fix of their likely arrival, SKETR 3. Then, as I suspect the arrival was not loaded, they proceded to the next and last fix in the box, KMSP. Overflying that, they continued on the heading they held to get from RWF to KMSP. They would have had to make a right hand turn to get to either GRB, EAU, or BAE, that might take them to KDTW on a deviated route. That didn't occur, therefore I conclude it wasn't a misloaded destination that caused the problem.
Also note the radar returns overlayed on their route. I don't think a dispatch-altered route would have deviated them so far north given the relatively benign weather they were experiencing.
Your thesis, filejw, might be plausible had there been an extended weather area to avoid, but that clearly wasn't the case on the night in question.