- If you mean "you cannot EXPLAIN AGI/consciousness/intelligence if you don't understand it", then that's true, but it's a trivial tautology.
- If you mean "you cannot DEVELOP AGI/consciousness/intelligence if you don't understand it" then that's very debatable.
Historically we have been able to develop all kinds of things, despite not knowing how they work. Tinkering and trial and error is often enough.
After all that's how evolution solved the problem of creating consciousness/intelligence. There wasn't some entity that "understood" intelligence that created it.
Sort of how we didn't have a "clear definition" of most things for millenia, but we still were able to recognize them as a class of thing. It's more of a "I'll know it when I see it" kind of thing.
And the external behaviors (of consciousness and intelligence) matter more than "but is it really conscious/intelligent inside" when considering some AGI as such. After all we neither can clearly define, not we know or can measure what's going on inside another person's head regarding consciousness, or to be frank, not even on our own head. When it comes to us, we just have a subjective experience, and not even a very clear one at that.
Well that's not true. We tamed fire before understanding combustion, friction, heat, or anything else.