You must have learned your grammar in publik skool.
Many of the delightfully described magical arts in the Harry Potter series were pretty standard fare in training courses I mastered to some degree or another
The writer is referring to mastery of training courses, not of telepathy and time travel.
Yes, but what were the training courses on? Telepathy and time travel, among other things. If the author really "mastered the training courses" for these items, she could have gone back in time and used telepathy to prevent J.K. Rowling from writing the evil Harry Potter books!