Yes, it's included.
First, a rifle's recoil system (buffer, tube, spring) has nothing to do with its operating system (DI, short-stroke piston, long-stroke piston); all kinds of combinations exist. Now, the Dissent's recoil system is housed entirely withing the upper receiver; it has no buffer tube. CMMG includes a 1913 (Picatinny) rail to be installled on a milspec lower where the buffer tube would normally be. You can use a folding brace or stock.