It's not just that most people are bad writers; people generally get worse at basic writing skills (grammar, sentence structure) when they are writing about unfamiliar or complex topics. When academics are reporting on their ongoing research, at least part of what they want to say will be ideas with which they have only recently become familiar, and the clarity of their writing will suffer as a result.

I actually disagree. Speaking from writing computer science papers, the moment of writing is when I understand it the best. I often have to go back and read my old papers to remind myself of all of the nuances of the work. I think a worse problem is the opposite: academics understand what they're writing about so well, and have been steeped in it for so long, that they can't "see" all of the information they leave out.

At the time, the writing is clear because I have had the entire topic in my head for a month. But years later, I can see I did not clarify simple things which would have made the text easier to understand.

I don't think the article conflates the two at all, it lists each out separately and in no way implies that they are the same thing, only that they are part of the same problem.

Except that the issue I have, and some others have, is that jargon is not a problem.