We can only speculate, ask, or maybe work it out when someone does a teardown. IIRC (it's been a while since I looked at the schematics - I may have mis-remembered) Khadas bypassed the XMOS in the Tone Board and routed the spdif from the coax connector and the pin header via some logic switches to the ESS DAC chip's spdif input pin. I don't know why they did it that way, but it may just have been simpler for firmware development, and they weren't doing anything like DSP in the XMOS. They did expose some other pins as I2S though, for an 8 channel expansion that never happened, and would have required new firmware to use them. IIRC a later firmware update used some of the pins for hardware volume buttons. Things like the MiniDSP MCHstreamer show the XMOS with other digital formats.