As it happens...
I think you're both wrong.
For EP, the paper and the reports say (almost) nothing about clathrates. This work is based on the thaw of pure CO2 snow.
For Chris, clathrates are favoured thermodynamically at any temperature and pressure compared to a mixture of CO2 and H2O ices, so if both ices are stable, then the clathrate is also stable, and preferred.
BUT, the formation of clathrate is kinetically unfavourable, except at high pressure, which is why we don't see it at the surface of Mars.
(not a neat and tidy answer, but sometimes they just ain't)