I can think of several reasons why freezing would not be a good idea...
I've not been there, so its a shot in the dark, but I'd expect any time below freezing 'not to count' as curing time.
Thats a very rough approximation.
Because the stronger the brine, the lower its freezing point. And the brine concentration (saltiness) will vary through the thickness of the meat during curing... so the freezing point of the meat will vary through its thickness, with the least cured being the easiest to freeze.
Saltpetre/nitrAte curing is rate-dependent on bacterial action, which slows right down around 40F/4C (thats why fridges run at those temperatures). But nitrIte curing isn't purely bacterially controlled... but does still rely on diffusion through unfrozen liquid to permeate the meat...
(and remember that freezing point depends on the salt content)...
not easy... As I said to begin with, not a good idea. I suspect the risk is rather uneven curing.