Putin has lied constantly during this conflict. He said his troops were not on Ukraine's border when they were. When called out on that, he said they weren't going to invade right up until they did invade. The latest is that Russia agreed to honour a limited ceasefire around civilian evacuation corridors but then broke that ceasefire.

Evidently, all we can do is respond to what Putin does. One must assume that, if the West declared war on Russia, Russia would take specific military actions. Since Russia has not responded to sanctions with those military actions, Putin evidently does not view sanctions as a declaration of war, no matter what noises come out of his mouth.

This is the downside of constant prevarication for state leaders. If Putin ever truthfully tells the West where there is line past which he will respond with force, nobody will know he's telling the truth. e.g. Putin has repeatedly implied that instituting a "no fly" zone would result in nuclear war, but it's clear we can't trust that. It may be a lie. If NATO planes move in Russian planes may simply move out with no further response from Russia.