For me one argument that tend to make me think that this is a false flag attack is how quickly they managed to arrest "all the suspect" involved in this attack. Usually when the Kremlin wants to act quickly that's because they want to wrap this up quickly so that nobody starts looking. Another such example is when the Wagner private jet went down, we could see how quickly the Kremlin communicated on this and how quickly they revealed the list of passenger that died on that plane.