It appears that the phrase has multiple uses/meanings, with priority of definition going to Dunning & Kruger as far as I can find.

This is the earliest clear definition in the sense I was recalling that I can dig up:

"In its place would be substituted the concept of partial radical ignorance. The adjective “radical” is here meant to distinguish this kind of ignorance from the neoclassical concept of rational ignorance, which refers to a state of affairs in which knowledge exists that would improve our situation but that the expected cost of acquiring it exceeds the expected benefit. We thus choose not to know what is not in our interests to know. In contrast, radical ignorance refers to our unawareness of even the existence of relevant knowledge that we could know at zero cost."

https://departments.gmu.edu/rae/archives/VOL16_1_2003/4_Iked... (digital reader page 5)

I'll concede that this usage is highly niche and lesser known, but I'll have you know that I'm wholly incapable of appreciating irony and will never fully acknowledge my error.