Its not a contradiction because Barba wasn't even playing FB for half the year, despite being signed to play there, and finished the year playing 5/8 and being at least the third choice FB behind Hodges and Hoffman. Boyd isn't taking his spot, he lost it before Boyd even came into the picture.
Bennett didn't sign him so I don't see why he owes him any kind of loyalty.
The coach picks the squad he believes can do the job he wants and with Boyd, like many players before him from Benny's time at the Broncos, Bennett knows what hes going to get from him and can depend on him to fit the role he wants him to play. Had Benny not lasted as long as he did at the Broncos there would have been a lot more boyd-like examples of players following him imo.
I can understand people not embracing Darius signing but I believe he has truly confronted a few personal issues and grown a lot, even in the space of a few months, and I think he will surprise a few people if given the chance.
Even in the short term we are better off both roster wise and cap, like I said what Barba could offer the club off the bench or playing ISC would likely be better utilised by at least a handful of other guys.