Because what some ruler calls him- or herself isn't really a useful category.
If you accept this definition, the USSR was an empire in the exact same sense the Romans or the British were, they just instantiated the template a little differently.
The ideals of the exceptional Republic make it the perfect overseer of the implementation of its system upon inferior nations that lack the vision or commitment to find and implement the perfect ideals. My impression from how it is coverred in the US is that the UK's early colonialism was simple feudal empire (the Queen is pleased with your venture) and late colonialism was of the Republican empire sort (they need our assigned administration to at least become a poor model of us).
That is a peculiar definition of "colonialist".
For instance, the ancient Romans colonized Great Britain, despite the fact that the Romans and the Britons were both "white". The Arabs colonized Spain, despite the fact that the Arabs were "non-white" (by some definitions) and the Spaniards were "white". The Mongols colonized a whole bunch of areas, of pretty much every ethnicity that exists in Eastern Europe and a large swath of Asia. And so on.
I had thought the revolution was to destroy all that, remove the czar, bring in autocratic states, etc.
I feel like there is a distinction here but I can't easily point to it.