Argument by turtles is only invalid if it doesn't converge in the end. If a professional designer needs to have less than half of a professional coder's expertise, and a professional coder needs to have less than half of a professional hardware engineer's expertise, etc., then the whole thing converges to a finite value.

An alternative way to view this: the output of your work in your field will almost always be priced and used outside of that field. The more you know about how your work will be used, the more context you have to evaluate whether or not you're doing things right. Given how human cognition works, having extreme tunnel vision is actually suboptimal, compared to being somewhat proficient in things around your particular specialty.