I think maybe how you are conceptualizing design and how the GP meant it are not in agreement, and if you came to agreement on what it meant you wouldn't really disagree about the point either.

For example, I think design, as they mean it, could be described as "how to get that thing we care about". The correct amount of design depends on how exacting the outcome and outputs needs to be across different dimensions (how fast, how accurate, how easy to interpret, how easy to utilize as an input for some other system). For generalized things where there's not exacting standards for that, AI works well. For systems with exacting standards along one or more of those aspects, the process of design allows for the needed control and accuracy as the person or people doing the work are in a constant feedback loop and can dial in to what's needed. If you give up control of the inside of that loop, you lose the fine grained control required for even knowing how far you are away from theoretical maximums for those aspects.