I genuinely don't want to sound pedantic or critical - but anyone who dabbled in maths - from olympiads in school or undergrad or did math for research - would have several such notes for e.g. of graph theoretic models or combinatorics doodles. I used to color mine, or had different dashes/bold schemes for visualization of my problem space.

I think blog author came across this & found it beautiful (good for him!). But IRL this type of doodling is commonplace among math folks.

From their viewpoint, most mathematician's I've met, admire someone's blackboard skills - handwriting, problem layout, & Hagomoro chalks. And the second one would be the witty/intelligent communication in their papers, book or seminal work - which sometimes gain cult status in their small circles.