Although I have no issues with what is written in the article, I think it misses some of the most important reasons experts are not always the best teachers. Two things I have seen myself related to instructional concepts called scaffolding and interference theory.
Scaffolding
New learning builds on old learning, and a good curriculum is carefully designed to take students through a sequence in which each step builds upon what went before, allowing students to transfer the learning from one to the next. Each stop puts a transfer load on the student. If the transfer load is too great, the student cannot succeed. If it is too small, the student is bored. During each transfer, the instructor provides the instruction and support needed to help along the way. That support is called scaffolding.
A good example lies in the typical OW class, in which the students go through the following sequence: 1) partial mask flood; 2) full mask flood; 3) remove and replace mask; 4) No mask swim + replacement.
I have observed classes in which the expert teacher impatiently wants the student to progress through the sequence too rapidly, without enough scaffolding to help the struggling student. We had threads on this topic several times in ScubaBoard's history, especially regarding tech training. A tech instructor I know told his students that "in tech training, we don't hold your hand," and by that, he meant "I'm not going to give you any actual instruction. I'm just going to require you to demonstrate the skill and then berate you when you can't do it." If allowed to use his approach in an OW class, I would bet that in the first confined water session he would just tell the students to take their masks off, swim around for a while, and put them back on.
Interference Theory
You can only learn so much in a given amount of time, and when you are learning, some of what you learn interferes with your ability to learn other stuff. In a well designed curriculum, the content has been carefully chosen so that the course ensures learning of what is essential at that level while still finding emphasis on what is important. That which is merely nice to know is de-emphasized, and that which students do not need to know at that level is eliminated. That way learning the less important stuff does not interfere with the students' ability to understand and remember that which is truly important.
All too often, the real expert wants to demonstrate that expertise by emphasizing stuff that is not important at that level of training, thus interfering with their ability to understand and remember that which is truly important. I remember being at a cave diving shop waiting for a ride at the end of the day, and I watched the first day of a Cave I class being taught outdoors. The class had been going on for about 8 hours at that point--maybe more--and the students had completed two training dives earlier. As the exhausted students sat there listening with glazed eyes, the instructor was lecturing them on the internal structure of the isolation manifold, explaining the mechanics that allowed it to work as it did. I have no idea why they needed to know that then (or ever, really), but everything they were trying to understand there was interfering with their ability to understand the truly important stuff they had been taught hours earlier. It is possible that this was actually part of the curriculum--in which case it was very poorly designed curriculum.