Gumilev wrote things that, in my opinion, were not very serious and leaned towards some unhealthy esotericism (these passionaries...). One of the main characteristics of an empire is that it is a MONARCHICAL state. If we deny this, we are simply going against the meaning of this word in the Russian language.
In China, there are indeed different ethnic groups, and the difference between them is no less than between the English and the French. And for Gumilev, fitting China into his theory was, I think, a big problem...
P.S. - everything said about Gumilev is IMHO.

Added after 34 seconds

Read Gumilev

I read it. It's awful, IMHO.
Read Marc Bloch instead.