Mutt,
All manufacturers and mod outifts inject confidence margins into their products. Even the FARs build a safety buffer into how balanced field length is calculated. Balanced Field Length is actually 115% of actual per the FARs. So right there you have 15% slop in the calculation. Add on the other 5-10% of what the manufacturer threw on there and your actual field length requirement is somewhere between 20-25% less than what you calculate.
In this case the runway is 8100 ft or so long. That means there was anywhere from 1600 to 2000 ft of slop built into the takeoff calculation.
I am not trying to say these guys made any mistakes by continueing the takeoff roll. That what we are tought to do. But in some cases if you had it to do over again, slamming the brakes on and maybe hitting something at a slow speed at the end of the runway was smarter then leaving the ground. The concorde crash was a good example of an instance where the plane should have never left the ground regardless of the consequence of crashing into something at the end of the runway.
The problem is, it's after the event when you figure out what you should have done.