This is avoiding an common but unnecesary round trip. When your inputs are vectors, angles are an unnecessary intermediate representation. You can substitute the geometric meaning of dot and cross product directly into the Rodrigues matrix and get by with less operations overall. It's more elegant, uses less instructions.
To the blog poster:
Robin Green is an excellent resource
Faster Math Functions:
https://basesandframes.wordpress.com/wp-content/uploads/2016...
https://basesandframes.wordpress.com/wp-content/uploads/2016...
Even faster math functions GDC 2020:
https://www.gdcvault.com/play/1027337/Math-in-Game-Developme...
And further to that. https://fgiesen.wordpress.com/2010/10/21/finish-your-derivat...