I have some info from Jeff… (slightly edited for English and clarity)
705K and 768K via usb is not supported in current firmware. But it [is] supported by I2S. We may release some firmware update to support 705K and 768K via USB later. But it seems not [many] user[s] need that. So the current plan is to support this at Spring2.
So it looks like XMOS chip is advertising 768k (which is needed, of course, for DSD256 DoP), but 768k is not supported. If you want it, email Jeff!
For linux, there is a white list embedded in the kernel. Only the USB id[s] listed in that white list are treated as DSD native capable. So that white list need[s] to be updated. We have added into this white list last year. So if it has the latest linux kernel. It should be able to report it is native DSD capable. Some device[s] (like auralic aries) bypass this white list.
This one (for native DSD support) is one for you @Dan_Knight.