I think there is lot of fair questions if such person is able to effectively write. And write in a way that enough people would be willing to spend time reading it.

Context is important. The blog discusses people taking training to be writers. Which I take means they aim to do it at least some way as career. And that puts lot higher bar for what they should be producing.

That is like someone aiming to be artist in visual arts never would even look at any other visual arts. Or understand the history and such. Probably not that useful education if you aim to be hireable or sell.

Yeah, I don't think anyone is missing anything from someone who doesn't know/like how to read, sorry.

There's other mediums to express ideas that isn't books if you are functionally illiterate.

Imagine instead the post was about people who want to make music but don't listen to it.

I literally cannot imagine being a musician who doesn't listen to music.

Having "difficulty, lack of motivation and concentration" for reading, but not for writing, doesn't make sense to me. It's far more arduous, and in the first place you have to re-read what you wrote unless you're throwing a stream of consciousness straight into the void/dumpster.