I believe Asrock is opting for an external power brick on this system. With the space, I doubt there would be room for one internally.
It might be taller than the Mini-Box M350, but it is still overall smaller. The M350 is a 2.5 L volume case, while this one is less than 2 L.
2L without the external power supply (which is not pictured, funnily enough)
Whereas the M350 can handle a Pico PSU up to 150W
Assuming you're running a newer i7 at 65W, M.2 SSD and RAM on the mobo (along with maybe 1-2 USB peripherals assuming you haven't gone full bluetooth) your power draw will be comfortably below that.
Both the ITX in M350 and STX in the asrock case will require a Thunderbolt 3 port for an eGPU to be mildly versatile and useful.
I want to like STX, but it's not adding any value vs ITX so far