This is much, much more than "just assumption" to Zizek. In fact, this is what he refers to as ideology and it's basically the main subject he is known for
The notion of unknown knowns strongly resembles the apocryphal Mark Twain quote; "It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so". (There's no apparent proof he'd said or written this.)
They just quoted the wikipedia article (without attribution). It, in turn, cites the link above.
It's hardly original to him. It's a negative contribution, as a lot of dimwitted people disavowed the idea of unknown unknowns after he said that. It's actually a valuable concept.
I find it a useful reminder at work when approaching projects with a team of mixed ability...
(I'll refrain from commenting on other aspects of his legacy.)
https://en.m.wikipedia.org/wiki/There_are_known_knowns