His Conclusions. The increase in average vertical velocity from 6,718 ft/min to at a minimum 8,623.1 f/min indicates a sustained vertical acceleration of the aircraft.
He assumes that the airplane was at FL350 at 2:10:10, which is possible but not an established fact. He then goes on to assume that the rate of descent between 2:10:10 and 2:14:21 was constant, which is highly unlikely. (I recall that an earlier calculation was in error but didn't check whether that has been corrected. The matter was discussed quite some time ago on part 1 of this thread.).