I think you are correct on the TAS thing as well. Your terminal TAS is higher at altitude.
Why would this make a difference?
Your terminal velocity is only higher because there are fewer air molecules to slow you down. Therefore there are equally fewer air mollecules available to create an opening shock.
Using Reducto ad Absurbum
what would happen if you bailed out in orbit?
In short, TV and any opening shock is dependent on IAS and not TAS.
I think the temp reason is more plausible, plus, don't forget they were jumping rounds, so if you open over the mess at 20,000ft and can see your labrador frolicking with the ground crew, with a North Wind, you'll still end up having schnapps for a night cap along with a couple of rousing choruses of the Horst Wessel song
Here's another reason: You might want to take a delay because your a/c was going faster than you will at Terminal Velocity. Now THAT could shred your canopy.
