It's because that little chunk of South America is French Guiana, which is a full-fledged part of France (not just a territory, but a departement with political status equal to the ones in mainland Europe).
It's also where the European Space Agency launches their spacecraft from, by the way.
Cool. I'm not sure what that has to do with the continent, though. Politically, I'm uncertain if that would count as something other than colonization, either.
It depends what aspect of colonization is important to your definition, I guess. Is it about imperialist expansion to non-contiguous territories, or is it more specifically about extracting resources from those territories for the exclusive benefit of the motherland?
At any rate, I would say that French Guiana either counts or doesn't count as a colony of France to the same extent that Hawaii counts or doesn't count as a colony of the United States.