That view seems a bit restrictive. There are cases where having more on screen is useful and actually helps readability, in particular for technical and scientific documentation.

Reading an article or a story is pretty much a linear activity: you progress onward and never have to go backward. In that case, readability is very much influenced by a page layout that reduces noise.

For other types of documentation, the surroundings of what you are reading are important, and having to actually move the page up/down or flip pages on a book makes you easily loose context.

So maybe this n-column reflow is not for everything or everyone, but if it was an option available in my browser, I know I would use it quite often.

Another thing to note: newspapers and magazines have multiple columns of text, and they are still very readable. So that n-column layout in a browser might not even be bad for reading articles on website that don't have busy side panels.