I felt the same. It is a decent enough refresher though!

Machine learning lends itself to easily learning additional pieces of math once you have a nice foundation, and it is nice enough that the foundation is pretty small---vector calc, (mostly) real analysis, linear algebra (it helps if you know infinite but orthogonal eigenfunctions), little bit of physics knowledge (statistical models and hamiltonians), and a little bit of differential geometry.

Unfortunately, that foundation is slowly getting bigger. Read some papers in topological data analysis recently. Category theory is crawling in!