Kennan's comment is strongly linked to the outcome of WW2. The US was the beneficiary of the war in sheer economic terms. Europe was either destroyed or bankrupt. China was devastated by it's war that predated WW2 with Japan, and then faced a civil war. The USSR was devastated as well, and had put almost all its resources into its military.

It's important to recognize the roots of the US's dominance post war.