"Islamic State fighters attacked a large gathering of Christians in the city of Krasnogorsk on the outskirts of the Russian capital, Moscow, killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely" says a statement translated from Arabic picked up by Visegrad 24, a reliable-ish pro-Ukrainian Twitter newsfeed.
What's semi-plausible about the claim is that ISIS has been more aggressive ever since Trump didn't defeat them in four weeks, and that bombing of the memorial for the Iranian Revolutionary Guard General Qassem Soleimani back in January. Whatever the Kremlin could gain by staging an attack on their own people and making up a claim that it was ISIS is not clear – or at least not clear relative to if they had faked a statement from some pro-Ukrainian partisans. So maybe it really was ISIS.
Before their leadership sold out the Kremlin the Chechens were in a long guerrilla war against Moscow, but they were not the only group. Countries with significant Muslim minorities spanning literally all the way from Nigeria to the Philippines have been home to ISIS franchisees for more than a decade, no reason to think Russia is any different in that respect. On the flip side there's strong evidence that Putin engineered a number of late 1990s-early 2000s attacks on Russians. Who better to assign the blame to than the guys who already had a history of conflict with Moscow?