This seems to be another expression of the truth that "people tend to mistake the limits of their view for the limits of the world."
1. You don't know what you don't know.
2. You know what you don't know.
3. You know what you know.
4. You don't know what you know.
They weren't "unknown unknowns". We knew. He knew too, but he was trying to muddy the waters. He got his way at the expense of one million dead Iraqi civilians, thousands of dead US soldiers, and 3 trillion dollars of US money.
I'd first encountered it in print in a 1982 book, Forced Options, by Roger Lincoln Shinn, two decades before Rumsfeld's famous 2002 usage:
<https://archive.org/details/forcedoptionssoc0000shin_c8x7/pa...>
- George Bernard Shaw, "Ceasar and Cleopatra"