Qt version in amber-phone supports only zxdg-shell-v6. Since phoc already dropped support of this protocol in favor of stable xdg-shell, it makes no sense to try to use Wayland backend there.
The only benefit it brings is non-blurry Qt apps under Flatpak, but at cost of breaking any native Qt apps. Although it likely doesn't matter much for the phone, it's a pretty big issue when it comes to convergence use cases.
Byzantium has a much newer Qt, so this isn't a problem there.