It may not be SystemReady, but it will capable of running a generic ARM image with a standard ESP setup. Though the catch is, you need to flash the SPI ROM first. This means either plugging it into a different machine, booting it up into firmware flashing mode (maskrom), and using rkdeveloptool to flash the image. Or boot their custom image off a microSD card or eMMC module and flash from there. You can do that now that on the Rock Pi 4 (running the RK3399). Everything has been upstreamed too my knowledge. Rockchip is currently in the process of up-streaming support for the RK3588(S), so it will be some months until a purely generic ARM OS will be bootable. Though in both their provided images, and in a generic ARM OS, it may be advantages to apply their provided device tree overlays (eg, switch the default PCIe Gen 1 to Gen 2. Support for some Raspberry peripherals).