Think its very hard to say why a ferret has died below ground,,, but its more likely to be through sufercation than cold,,, very low oxygen levels below ground...
And the ice they kept below ground was in ice houses,,contsructd of brick or stone,, and covered in earth,,, and they usually kept tones of ice in there collected from the frozen lakes in winter,, part of the reason it stayed frozen was because the way the ice house was constructed,,, and the amount of ice kept there....
Having said all the above,,,,i think its quite possible for ferrets to die of the cold,,, but I think they