If China is "hemmed in", why are so many people so concerned over what China is up to in Africa, or Central Asia?

I'd suggest that the traditional idea of geographical containment is no longer particularly relevant, and neither is the idea of a sort of physical "perimeter" on a global scale.

The question of why we would or would not fight in any given place or case is always interesting, especially since it largely depends on domestic politics at any given moment. Again, though, I don't see how it relates to this idea of an empire, nor have I seen any credible definition of "empire" that would accommodate the US.