This isn’t terribly surprising. He was non-guaranteed, and at more than the minimum salary (or, more than the minimum salary they can replace him with.) He had to win a rotation spot in camp to be kept, and not merely be in the top 12 or 15 players.
Look for the C’s to sign a player or players to a non-guaranteed 1-year deal, as needed for depth issues. We’ve heard of a big being brought in to start the season due to depth, and I could also imagine a PG a being signed later in the year of depth issues strike there instead.
If someone they sign plays well, he might be kept past the January guarantee deadline, but I expect them to be at 14 players in mid-January, giving them trade deadline flexibility, while also making it easier to reduce or eliminate a tax bill if the level of competitiveness calls for it.