"Burn testing" does not imply "actively mining at extended length" IMO.
It implies a brief period of testing to ensure the device works and doesn't fail within a few hours.
You say tom-a-toe and I say tom-ar-toe.
We cannot say how long something is burned in for, we can only say they were used on EMC contrary to what BFL had said previously about doing the burn testing on the Testnet.
Personally I'm inclined to think a lot longer on the basis of BFL purchased EMC (and kept quiet about it) and all devices are set to run on EMC by default (via Easyminer).
Whilst there is no evidence to support either a long or a short burn in testing phase, I know what I (and presumably many others) think.
The document does state explicitly "Butterfly Labs earns mining income from their burn testing of machines", so why would they limit how much they earn?