He means its the last card the west can play short of war. If they unleash all of their cards now they have no chance at dealing with Putin in the next few days. As small as the hope sounds, its possible Ukraine could resist enough and possibly hold some ground, and Putin could be convinced to stop his advance at some point with the threat of swift being revoked.
Our cards won't change in the next few days. If playing them now or a few days ago wouldn't have stopped Putin, why do you think those same cards would work better once Putin has gone even further and owns Kyiv?
