yeah you can't get the upgrade until the game releases, then you'll be able to "buy" the free upgrade and download it. You can't ever preorder upgrade vouchers, paid or free. Uncharted Legacy had the same thing. the good news is the PS5 version should be 1/2 the size of the PS4 version
and yeah on the PS5 console itself you can't buy the PS4 version, have to use web or mobile app lol
a lot of it was because Sony didn't actually want to offer a free upgrade, initially it was gonna be $10 fee but people got super mad and Sony walked it back since HZD2 was supposed to launch in the window when PS5 upgrades were all free (that window is over)