CBC-MAC insecure with random IV
Then, the tag (m,t) is a valid MAC for the message IV.

In other words, given a tag $(IV,t)$ for a message $m$, you can construct a tag $(m,t)$ for the message $IV$. So your understanding is wrong. It is not $m\oplus m$ in the forgery, it is $IV\oplus m$ which is the same as $m\oplus IV$.