This is just silly. The author of this article is perpetrating a logical fallacy of equivocation revolving the multiple semantics of the word "impostor".
Impostor syndrome is the fear that one is an undiscovered incompetent, promoted into their position by luck.
Impostor syndrome is not the fear that one is a literal impostor: someone who consciously takes on a false identity and lies to get what he wants, yet is somehow not aware of this! (That would be pretty serious psychiatric disorder!)
Impostor meaning "incompetent person in an undeserved position" is different from "someone who lies and disguises their identity to gain what he wants".
Of course impostors of this second kind, the conscious liars and pretenders, know what they are. That is completely irrelevant to impostor syndrome.
Impostors, the incompetents, often do not know they are incompetent. This is known as the Dunning-Kruger effect.
However, incompetents sometimes do know that they are incompetent.
Whether or not you think you are incompetent actually has no bearing on whether or not you are. All four combinations of think/are are possible.
If you suspect you're incompetent, you've avoided the Duning-Kruger effect; but that doesn't prove you're competent. You simply could be right.