Sounds like you can't use the voucher and, say, a parkhopper ticket in combination to get a multiday ticket or an annual pass. But I'm no expert. I'm sure the experts will be along soon...
Yes, that is what it means, although there have been a couple of people that say they were able to combine their parkhopper and GAD voucher to upgrade to the AP. Probably all depends on the CM you get.
For what it's worth, I have heard that people would able to apply an UNUSED parkhopper towards an AP, same as cash.
So if you had a GAD voucher and a 3-Day Parkhopper, then used the GAD voucher to enter the park (without using the parkhopper), you could in theory exchange the unused hopper and your one-day ticket to upgrade. (If I were doing that route I would probably want to call Disneyland to confirm.)