Thanks. Very useful link.

Single-column would require the font size to be increased to keep to optimal line length (https://baymard.com/blog/line-length-readability), and increasing the font size would require more paper when printed.

I like multiple columns. There is a reason that newspapers and many magazines have settled on that, and it's about readability. I wish more websites would do the same. What is your issue with two columns for papers?

I have a slight color deficiency, so the sample formula and paragraph are a bit hard to read for me, especially the paragraph. Up to 8% of male adults suffer from this, at various levels, so I'm surprised the authors did not mention this problem.

Another point is that their choice of colors seems to mix various luminosity levels. So I suppose that, even for someone without color blindness, some colored texts are are much more visible than others. I doubt this hierarchy was intended.