If Roon were downsampling on the fly, it would have shown up in the signal path.
This is my speculation based on a little bit of google and may well be wrong: I suspect later revisions of the DAC actually supports DSD512 but the manufacturer does not advertise it, possibly due to support / driver reasons, or don’t want to make early customers unhappy due to hardware revisions.
Other users already confirmed DSD512 worked:
As for why W20 limits to DSD256 but ropieee works at DSD512, the latter is using native DSD while the former is doing DoP.