the topic of porpoising wasn't even mentioned to" him during his
entire training process


Which may, or may not, be true. Student pilots have lots and lots to
learn, and unless an item of knowledge is imparted multiple times by
the instructor, it's not likely to be retained.

What's the Chinese proverb: I hear and I forget, I see and I
remember, I do and I understand.

The vast majority of study that is not constantly reinforced by flight
activities is forgotten by most pilots.