```
This attack might sound a little academic, and far from a practical
problem. We may have proven that the MAC is insecure by our
definition, but the only signatures the attacker can successfully
forge are for modifications of messages that we sent and signed:
specifically, the message we sent, followed by some binary junk,
followed by something the attacker chooses.
```

The extensions are still modifications; the "specifically..." part says which kinds of modifications. If you leave that part off, you might interpret that sentence as saying the attacker can forge MACs for any modification of the message, but that's not what it says.

(I'm also modifying "signed" to be "authenticated", so that there's no confusion with signature algorithms)