Yes, it's not that people know specifically what they don't know, it's that they develop the wisdom to know those boundaries and anticipate them and reduce their likelihood and impact.

For example, if I use the language of my expertise for a familiar project then the boundaries where the challenges might lie are known. If I start learning a new language for the project I won't know which areas might produce unknowns.

The LLM will happily give you code in a language it's not trained well on. With the same confidence as using any other language.