I presume he means he has the mathematical aptitude to teach high-school math, which (at least in my area) maxes out at trig in most schools and Calc 1 in the bigger ones.

To teach as an adjunct, that's about all you need to teach. If you want to be good at it, some genuine compassion for your students will help there. If you want to be awesome at it, you'll typically need the above plus some training in the proper ways to educate, motivate, etc.

Source: married to a college math teacher.

More easily than a teacher who, like we kids always suspected, actually needs the answer key?