What the article doesn't emphasize enough:
Pinning of the contact line is crucial (e.g. due to surface roughness), otherwise the ring would not be as pronounced. Due to higher curvature, evaporation is faster at the edges, causing the non-evaporating solids to flow to the edge leading to more of them there in the end when everything has dried up. But on a smooth surface, droplets just shrink. When they don't, you get the ring stain.
It is a bit annoying that the article does not link any relevant research. There is a wikipedia page on the topic (https://en.wikipedia.org/wiki/Coffee_ring_effect), but afaik it is an interesting problem in many different contexts, for example in inkjet printing (one can find plenty of articles there as well).
This is the original seminal work on coffee drop evaporation out of Sid Nagel’s lab, with theoretical support from Tom Witten and Todd Dupont and their students.
Like everything out of the Nagel lab, at least from that era, it combines a keen curiosity about things we take for granted with rigorous physical experiments and insight.
The Nagel/Witten collaboration was one of the many lovely things at the University of Chicago in that era, and it was always tremendous fun to see them present and get a glimpse at how they approached problems.
It was like looking over the shoulder of giants: often humbling and always educational.
Relevant: https://www.nature.com/articles/nature10344