No idea TBH. I'd assume that the water bottle is food grade and it can take the weight of all that water, but I've never heard of anyone using them as a fermenter before, probably a reason for it.
From the spiel on their website;
"BetterBottle carboys are: 1) Designed to be strong, scuff resistant, easy to handle, and essentially unbreakable, 2) Pure Taste- and odor-free, BPA-free, DEHP-free, plasticizer-free, 3) Virtually impermeable to oxygen, 4) Extraordinarily easy to wash and sanitize (no brushes necessary), and 5) Clear and colorless. BetterBottle carboys are not at all like other plastic carboys (Use the FAQ tab to access the BB vs. Plastic discussion for additional information). The special PET, used to make BetterBottle carboys, is non-absorbing, non-porous, and non-wetting (hydrophobic), so it will not carry over flavors from one batch of wine or beer to the next and it is easier to clean and sanitize than glass or other plastics. BetterBottle carboys are ideal for home winemaking and home brewing."