There is no “smoking gun” tying this event to the CIA or any Western intelligence agency. You must ask, who benefits? Not Ukraine or its Western allies. Increasing Russian resolve to defeat Ukraine is not in their interests. If the Russian media claims Isis is a tool of the West, that will have the effect of increasing that resolve. Just like 9-11 did in mobilizing the U.S.
As I have said, false flag or allowing it to happen both favor Russian strategy as you stated.
Tying attack to west would be a huge benefit for internal consumption and support for the war