The difference is that when you're buying a x86 system, the entire CPU bringup (incl. AMEGAS/openSIL on AMD) runs proprietary and poorly documented firmware. You're entirely at the mercy of the vendor.

Oxide has put immense effort into writing open-source platform initialization code, and built their own open-source BMC/RoT solution.

The CPUs are x64 but the architecture is not that of a PC, there is no BIOS, etc. You couldn't boot Windows or Linux on the bare metal. The hardware, firmware and hypervisor are custom built for control, safety and observability. On top of that, the application OS all run on VMs which _do_ have a (virtual) PC architecture.

The CPU is commodity, nothing else is. Costume Mainboard and firmware without BIOS and their own BMCish thing and their own Root of Trust. Same for their router. Standard chip, everything else is costume.