I understand that Putin dismissed US warnings in a speech three days before the attack. However, the warning had been for a 48-hour period, on 7 March.
Maybe the attack didn't happen during the 48 hours because of tight security?
let's follow the logic of what you are suggesting.
The US uses the 'wrong channels' and warns of a terrorist attack the weekend of the 8th. Russia puts in place security which deters the terrorists. Russia decides after the weekend of the 8th, they are safe. Putin a week and a half later declares they are ignoring the US warning (which at that stage is a lie by your suggestion). The terrorists decide after Putin says this to carry out the attack. People are murdered.
So by that approach, Russia are lying, incompetent, and incredibly stupid.