Walter makes a valid point.
I would imagine an aircraft flying at 70 IAS / TAS with 100kts tailwind and an aircraft flying at 170 IAS / TAS in still air would create very similar debris paterns on the ground.
Mr Holbrook could never assess IAS / TAS he could only assess GROUNDSPEED.
Even if there was a sudden and dramatic increase in tailwind of say 30 kts over the cliff ( which incidentally would lead to severe turbulence) it can only account for part of the increase in GROUNDSPEED.
Some of the increase had to come from increased power.
I have never met any pilot, who, confronted with deteriorating weather conditions, deliberately increases speed.