For me, "Those who can, do" means that people who have a natural talent express it easily through endeavors -- painting, music, mathematics, engineering, counseling, gardening, whatever. Someone who handles difficult tasks without much effort is usually a topnotch performer, but not necessarily a good teacher of the trade.
Earl Scruggs, a musician, invented a style of three-finger picking that has spread across the world and is known as Scruggs-style banjo. He is, of course, one of the best purveyors of that style, but his book on how to learn it doesn't explain the steps very well, at least not for me. Perhaps people with some natural affinity for the skills needed find the book useful. It demonstrates how Earl plays songs and has an excellent history of the instrument, but offers no clues about overcoming difficulties while learning. I suspect Earl had few problems and thus can offer few solutions.
Pete Wernick wrote an excellent book on bluegrass banjo. He writes well and has a Ph.D. so he knows the many ways people learn. But I suspect he also had problems learning to pick, difficulties he worked though on his own. That knowledge helped him write a first-rate teaching guide.
For myself, I used to be tone deaf. After years of effort, I can play and sing reasonably well. I am also good at teaching melody since I had so many problems learning it. My students get multiple ways of figuring out a tune and calculating the best way to play it on the banjo. But after hearing me at a jam session with rock and rollers, one of my students asked how to syncopate a backup rhythm. I showed him but when he couldn't follow, I had no tricks of the trade, no special secrets discovered during experimentation, no alternative approaches to offer. I have my own rhythm that fits with most styles, and when it doesn't, I'm stuck.
Albert Einstein, the physicist, is often offered as proof of both a doer (Nobel Prize) and a teacher (excellent books on relativity theory). The pundits overlook the fact that the thought experiments dealing with the relativistic effects of high speeds were difficult for Einstein. He spent many hours contemplating them, working through the ramifications, and figuring out what the universe would look like if you were riding a light beam. After solving the problem for himself, he was able to help others get past the same dead ends he had encountered.
There are probably some people with natural talent who are also talented teachers. And while I suspect teaching itself may be an innate talent, especially useful for the basic skills taught in elementary grades, I certainly don't believe a natural teacher can teach any subject using a workbook. I would much rather learn from a journeyman who experienced troubles learning the skill than from highly talented professionals who never ran into difficulties with the subject. They have no empathy for problems in achieving competency. In fact, I suspect those types of people are the ones who give college professors a bad reputation for spewing obscure, irrelevant facts while arrogantly lashing out at uninformed students.
Those who can't naturally do but persevere anyway with a strong desire to master a skill can probably teach us all a thing or two about learning, working through difficulties, slogging through the low points of life. I remember coaches saying, "Ya gotta want it." People with natural talents don't "want" in the way the rest of us do. They can easily master a skill. But luckily for most students, "Those who can't, teach."