It is mentioned on page 10 but I have no idea what it is. Also, why not adding the rules to the PDF instead of redirecting the reader to another place where it seems like they’d have to pay (that’s my assumption from the line “Let me know if you need help buying a copy”).

Wonder & Wickedness, a popular set of freeform casting rules, available as a booklet on DTRPG, I think.

yeah it's Wonder & Wickedness (the name of the class ability) which is a cool book! I've been using it as my magic system for a long time. https://www.drivethrurpg.com/product/145647/Wonder--Wickedness