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"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 better hope those legal documents which seem to imply their burn in was well above a few hours are wrong

I concur that the miners were probably only tested for a few hours at their other facility (come to think of it it might have been at butterfly labs dedicated hosting companies). We put them in vans. Several days later the miners arrive at the testing facility. They are tested for a couple of hours. Then they are put back in the vans and delivered back to the plant arriving several days later. Reason? The traffic is terrible in KC. Oh My!