An emperor is a monarch, the sovereign ruler of an empire.
Term Empire or Kingdom is written with same characters in Korean language which is 왕국과 and this is the word used in Red Storm manhwa , so translators often translate kingdoms as empire or empires as kingdoms and we really have no way of knowing which term is correct when referring to certain nations with that word. IE: Rojini Kingdom or Rojini Empire. However, we figured we will call them by what they call the ruler, either king (왕) or emperor (황제). In case of Silence Empire, title of the ruler is "Emperor"(황제) hence, Silence Empire.
There is another word for Empire which is (제국) however, it's not being used often, hence the confusion. One of the examples of that word being used is in chapter 214, when Winnie and Shubeon are talking about empires culture, in which case it's clear that it's empire, not the kingdom.
This could use some clarifications by expert translator.