From the page you reference, it says that it may not be possible to port a number back to GV. Eg:
"Can't transfer Google Voice number
If you don’t see the option to transfer your number to Google Voice, it’s likely because Google Voice no longer supports this number or numbers in this area.
Google Voice’s number availability changes regularly, so even if your number was previously on Google Voice, it can’t always be transferred back. If you’d like to keep this number, we recommend transferring it to another carrier that supports it."
