If a designer thinks learning a little bit about how things work under the hood is somehow going to make them worse at designing, then they ought to also become worse at designing if they learn about anything else outside of their craft, like cooking, rock climbing, french, knitting, cars, football, or any other hobby or side activity.

Better stay completely incompetent at everything in life except graphic design. To focus, right? Yeah, it doesn't make any sense. The orthogonal skills we acquire don't prohibit us from deepening our existing ones, it's the other way around, really.