What was exactly his thinking here? Do biology teachers study biology because that’s kind of the first things you learn (saline concentration, kidney, etc…)
At least where I am, substitute teachers don't have to study anything in particular at all. You simply need to have completed X number of credit hours at an accredited institution, no actual degree or certificate is required and the coursework can be literally anything. As a newly-graduated history major I signed up to substitute teach out of desperation, and was sent everywhere from kindergarten to calculus.