Isn't the fact you point out the point the author is trying to make? The potential harm to Russia that would result from direct NATO involvement is a very potent bargaining chip in negotiating a settlement but if Russia calls the bluff that is bad either way. But if that threat is not there Russia has little reason to negotiate. I think the US could force an agreement by having both sides give roughly equal concessions they would not be happy with and threatening Ukraine with a withdrawal of support and threatening Russia with direct NATO involvement. The potential harm from the threat might be worse than the agreement so they might both go for it but only if they believe the US would carry it out.