Elsewhere it says:

"When you use Dictation, your device will indicate in Keyboard Settings if your audio and transcripts are processed on your device and not sent to Apple servers. Otherwise, the things you dictate are sent to and processed on the server, but will not be stored unless you opt in to Improve Siri and Dictation."

And:

"Dictation processes many voice inputs on your Mac. Information will be sent to Apple in some cases."

In conclusion... I think they're trying to cover all their bases, but it sounds like things are processed locally as long as the hardware can handle it.