Why would the Russians believe anything coming from US Intelligence? We’re not exactly Allies.
Generally speaking, up until now even normally opposed intel services would pass on information about suspected imminent attacks by terrorist non-state actors to their opponents. This is sometimes called “Duty to Warn”. I doubt this would hold up in a case where the parties were engaged in large scale open / direct combat, but short of that, it’s part of the effort to keep terrorism (real terrorism, not the overused term type) outside of the rules of war.
Of course, this begs the question of whether Russia is so deluded as to consider itself engaged in large scale open / direct combat with the US, on a level of desperation such that we would use literally ANY means for a small advantage. More likely Russia is just engaging in propaganda, but doing it here is at best foolish of them. No sane person doubts that Radical Islam is a serious problem for Russia too...