It quite clearly isn't because it's already happened. It has been stated quite a number of times by quite a few people that this isn't the case and clearly, it has already happened. There's no way the two Russian battalions in the Ukrainian defence forces just went out on a whim and invaded Russia, and not just the once they did it three times, without approval from the Ukraine military. There have been Ukraine attacks across the border throughout this war.
WHat is the difference between a raid / skirmish and an invasion?
Imagine the situation on the north front where the Ukrainian army to fight back against Russia, can't move north into Russia and go around the RUssian troops to encircle them, that would be a ludicrous idea to prevent.