Hash Fast guaranteed delivery by 12/31 or they would provide a full refund in BTC. They also said they will guarantee your BTC. I think they made those promises prematurely, but it was those promises that prompted many to take the gamble on their hardware.
We don't have a word "full" in writing. There's a word "payment" in the original TOS. HashFast is interpreting "payment" as a "BTC equivalent of the US$ value of the product". So they are offering that US$ amount back, in BTC, at todays BTC/US$ exchange rate. To get more than that (original BTC amount we payed) we have to fight them in the court of law.