That's legally true, but I think it misses the distinction between formal political status and the legacy of colonialism. French Guiana is an overseas department of France, and its residents have French citizenship and constitutional rights. But that doesn't mean the historical relationship was equivalent to mainland France, or that the effects of that history disappeared when the legal status changed.

The same point applies elsewhere: Guinea became independent from French colonial rule in 1958, but independence didn't magically erase the economic structures, institutions, and inequalities inherited from colonialism. Today, a large share of Guinea's population still lives in poverty despite the country's enormous natural-resource wealth.

So "it's not a colony" is correct as a matter of constitutional law. But using that fact to imply that colonial history is irrelevant to present-day economic and social conditions is a much stronger claim, and one that's harder to defend.

It's a colony even if it's legally part of France.

(Had to check: free movement does apply to Guianans, but in the other direction it's not part of Schengen)

For what it's worth, Metropolitan France used to claim Algeria wasn't a colony either.