Doing a bit more digging, it's using cytoscape[0] which is similar to graphviz. If you search your favorite search engine with "Cytoscape Session Viewer", you'll find many websites displaying the same type of graphs (select layout: circle).

FYI, circo doesn't always output a circle. I recently created https://github.com/MegaManSec/SSH-Snake/blob/main/tools/SSH-... using circo.

[0] https://cytoscape.org/