From the website. Basssically, yes they do allow more than glass, but it is an insignigicant source compared to stoppers, airlocks, racking.
BetterBottle PET carboys are made of plastic; however, they are not like other plastic carboys. First of all, they are made of a special PETand secondly they are made in a manner that packs the plastic molecules very tightly and creates a delicate balance between microcrystaline and amorphous regions.1 They are especialy well suited for home winemaking and brewing. So, why do rumors that no plastic is acceptable persist here and there? In the first place, many people have experienced failures attempting to use carboys made from types of plastic that are known to be too permeable to oxygen and to scalp flavors (see Flavor Scalping). Secondly, the mistaken belief that all plastics are the same is occasionally reinforced by sporadic reports of failures involving BetterBottle carboys. Making a good wine or beer is an art and results do not always meet expectations, regardless of whether the winemaker or brewer uses a glass carboy or BetterBottle carboy.
Yes, BetterBottle PET carboys are slightly more permeable than glass; however, it would be a mistake to assume that using a glass carboy will guarantee superior results. The traces of oxygen that penetrate BetterBottle PET carboys are incredibly difficult to measure and insignificant when compared with the amounts of oxygen diffusing through, or leaking past, traditional, liquid-filled air locks, traditional rubber stoppers (especially silicone stoppers), and most common types of flexible tubing. Moreover, oxygen diffuses into, and reacts with, wine and beer so quickly that removing a closure from a carboy, even briefly, for testing and making adjustments can allow a relatively large amount of oxygen to enter the small volume. Wine and beer essentially suck up oxygen. And racking from one open carboy to another open carboy with a siphon, a pretty standard approach when glass carboys are used, will add a great deal of oxygen in an uncontrolled manner.
The uncontrolled, excessive entry of oxygen is the real problem, because the addition of the micro amounts of oxygen, depending on the wine or beer, is generally beneficial.2,3,4 Wineries that ferment many hundreds of gallons of wine in a single tank may actually prefer polyethylene tanks, because the volume of wine is so huge compared to the surface area of their tanks. Polyethylene, or other similarly permeable material, cannot be used to make fermenters for small batches of wine and beer, because the volumes are small compared to the surface area of the fermenters. Using a BetterBottle fermentation system, even a novice will find it easy to rack or bottle cleanly under conditions that control oxygen exposure (Use the Products tab at the top of page to access How-To Tips.