Sure, in history they were considered separate. But again, that is true of every identity on Earth. Before the French became French, they were a hundred different tribes. Same with every other identity out there.
The difference is that the Manchus and the Mongols became Chinese. Manchus today speak Chinese and follow Chinese culture, while the Manchu language itself has almost become extinct. Today there is no difference between them and the Han, they have successfully been assimilated like many before them, including my own ancestors.
The Europeans on the other hand did not assimilate into the countries they colonized, in fact it was the opposite way around. They forced their language and their culture on their dominions, while never becoming native. Today they still speak only their own language and follow their own culture, and their former colonies speak their language too.
European colonists never assimilated into the host culture, instead they replaced them, like they did with the Native Americans or the Australian Aboriginals, or forced their culture onto their colonies without reciprocating.
