yeah - I wondered that as well................... page 257 explains:-
First, we look at DCA missions flown by U.S. and other coalition forces to counter Russian, Syrian, and Iranian airpower in Syria. DCA is defined as “direct (active and passive) defensive actions taken to destroy, nullify, or reduce the effectiveness of hostile air . . . threats against friendly forces and assets” and includes flying missions, such as CAP.1 For this category, we first analyze how the coalition met the emerging DCA requirement, which increased
greatly following the entrance of the Russian Air Force into Syria in October 2015, and how this requirement evolved during the operation. Although the coalition interactions with the Russian Air Force and the air-to-air incidents that occurred in Syria received a healthy amount of attention, the extent of the DCA mission in OIR tends to be an underappreciated aspect of the campaign."
later:-
DCA
ISIS did not possess an air capability beyond small UASs that it used for tactical reconnaissance and attacks.19 However, there was still a substantial DCA requirement for OIR, particularly after Russia intervened to support the Bashar al-Assad regime in Syria’s civil war in September 2015. The arrival of advanced Russian combat aircraft and SAMs created a potential threat to coalition aircraft. Such capabilities enabled the Syrian Army and Air Force to successfully go on the offensive against their opponents, increasing the chances for deliberate or accidental encounters between Syrian regime forces and those of the anti-ISIS coalition.