I don't think it's that extraordinary a claim to make myself. It's 1 lap in a low powered car. If the Stig showed you the best line, and you had 10-15 laps to perfect it, find the cars limits I don't see why not. These F1 drivers are not magicians, they only have the same power as you. If you find the cars limits and listen to the Stig over 1 lap there's not going to be much difference between your time and Hamiltons if your a decent driver. To make any judgements about his ability from that is a little OTT IMO. It's in high powered vehicles over a number of laps those guys are good.
Getting the right line, speed, braking, handling, changing gear is not as easy as it sounds mate.
I consider myself to be a decent driver and have tried my hand at track driving.
Its not a simple as you expect, you brake in the wrong place and your off backwards.
You hit a corner too fast, you lose power for the exit.
Select the wrong gear at the wrong time, can lose 2-3 seconds.
To say these F1 driver cannot be judged by this lap in a loew powered car is a bit naive really.
Do you not think Clarkson would think that too?