I think they use Android 11 on purpose. This is a retro gaming machine and some emulators don´t play nice with scoped storage that is forced on Android 12. I wouldn´t really blame them for using Android 11 as long as they keep supporting the device and fixing bugs.
Retroid has introduced a new version of the Pocket 3. Sold as the Pocket 3+, the gaming handheld has a 4.7-inch display, a Unisoc T618 chipset and 4 GB of RAM but only features the outdated Android 11.