I suppose that is true to some extent. However, if an artist is able to better understand things like spacial relationships, contrasting elements, color theory, and creative techniques you cannot deny they are likely better suited for the task. I won't' say it's impossible, but why would such a design critic as yourself be so swift to encourage one who arguably has no art background to try their hand at designing?....
I'm not encouraging anyone to do anything.
One can have a complete understanding of all of those things you list and still not be able to draw a stick figure.
Just like anyone can go to school and study music. Learn all of the notes, progressions, modes, harmonies, rhythms, buy an instrument and play notes upon it. But all of that is no guarantee that they'll be able to make music. Understanding the theory, tools, rules, and various elements of something is only coincidentally related to actually being able to do it. Whatever 'it' might be.
Back in the day I wrote code. Not so much for PC's, but on the big iron. I was paid an astounding amount of money, even by today's standards, to ply my trade. At the time there were but a handful of people on the planet capable of writing commercial grade code at this level and little Billy Gates wasn't one of them. I never went to school, never even took a class or a seminar although I used to teach a few. There was no such thing as 'Computer Science', we of my generation invented that, miserable thing that it turned out to be. I just sat down and did it. Then and now for the life of me I couldn't tell you how I did it, It just sort of flowed out of my fingertips. No diagrams, no flow charts, no outlines, no notes, no nothing, just code.
Some are able to just sit down and do it. Others not so much. Be it music, art, code, or whatever. This ability, or lack thereof, doesn't seem to have any sort of causal relationship with previous education in theory and tools. Certainly there are people with both theory and ability but examples of either one without the other are sufficiently plentiful as to call any necessary connection between the two into serious doubt.
Conversely, being able to whip out a rendering of something is no indicator of any ability to do sign and related work. Witness the all too common "I've got a nephew who can draw" syndrome that plagues every journeyman sign writer from time to time. While little Rollo may well be able to sketch and paint, he always seems to lack even a glimmer of understanding of typography and synthetic design. Likewise the most of computer science grads who couldn't code their way out of a paper bag.