This is commanding someone 'don't' isn't it? Seems strange to me. I've only ever used 'ddim yn hoffi' for I don't like. I've never heard peidiwch hoffi.
Peidiwch, for plural or formal uses, and Paid, for singular, are the correct ways of using the command, 'do not/don't. 'Ddim yn hoffi' is not correct as the command, it's part of a negative sentence, ie 'dw i ddim yn hoffi' etc.
She's saying that Peidiwch hoffi seems like a strange command and seeing as it wouldn't be used to say "I do not like" because that is Dw i ddim yn hoffi, it's therefore an odd sentence to have on the course.