FWIW I agree that the line quoted by philr, of itself, would have misled me into thinking that DoP extended to DSD 512 and that Windows was only required for native DSD. It would have been better expressed as:
DSD support: DoP DSD 64 to DSD 256, native DSD 512 via Windows.
You are right the DSD 256 limit over DOP is not clear. I had not read the whole of this thread carefully enough before responding. Chord are at fault here. I still maintain that a dealer who did not know this or did not understand this enough to get the right answer from Chord and Roon is not worth dealing with.
DoP512 requires 1411.2kHz PCM, regardless of brand. But the vice vesa is not necessarily true, i.e. support for 1411.2kHz PCM does not imply DoP512 will work.
A far more common case is when a DAC advertizes DSD256 with PCM 384kHz. Since DoP256 requires PCM 705.6kHz, it implies that the DAC cannot cannot support DoP256, and DSD256 has to rely on native DSD, which necessitates Windows with correct drivers or support by Linux native DSD.
I would like to thank everyone for their comments while I went through this process. Yes, sales people should be better informed, but it took me several weeks of active investigation to really get to the bottom of this, so equipment manufacturers and people that provide the hi rez music also share some responsibility.
If DSD were a bigger market force, vendors would have already addressed this problem by developing software for non-Windoz devices to support Native DSD instead of the encapsulation of DSD in a PCM envelope. Only time will tell. There is a good reason for developing the software though since Native requires fewer computing resources.