The most important parameter is not mentioned....Most silicon-based class-D amps operate at a 500kHz...
I downloaded their "whitepaper" and they switch at 400 kHz - like you, I can't see much benefit at that frequency.
The measured performance is typical of that sort of implementation, competent but looks well short of a PuRifi400 or nCore400.
They claim better efficiency in the output but don't explain much about why, perhaps
@DonH56 can expand on his comment?
A pity there's not more educational information in the paper to justify the, admittedly small, inconvenience that you can't just read it, have to fill in details and email.
Best wishes
David