Simply not true. Dan Campbell didn’t want to let him go at all, and the GM didn’t either. They offered him the same deal Montgomery got. The only reason they even offered Montgomery was because Williams was taking a while to decide and they didn’t want to lose out on both him and Montgomery.
I’m not saying you’re wrong about him being an average back or anything like that, but the Lions tried to bring him back and certainly weren’t happy to let him go. No matter if Williams felt they low balled him or not.