OK, I guess Shock isn't a defined physical term anyway which is probably at the root of this issue.
Since drag is proportional to IAS I'd agree that the peak acceleration will not change with altitude.
However, the faster moving object (TAS) will take longer to slow down, so the deceleration profile will be 'stretched out', in regards to time, at higher altitude.
Given that injury is related to acceleration and duration, then altitude has to be a factor.
I know sweet FA about parachutes, but basic physics tells us that surviving a rapid deceleration is all about transferring Kinetic energy to some other form. If your TAS is higher, thats more Kinetic energy, and therfore it HAS to be more of a problem. Regardless of the actual mechanisms involved, and regardless of whether the Parachute or the person is the limiting factor.
Using Reducto ad Absurbum what would happen if you bailed out in orbit?
mmmm... tasty. OK, I'll bite.
Well, with density zero, you are already below your terminal velocity. And your terminal velocity with a parachute is still fatal !
But lets say we fire a retro rocket to get us back into the upper fringes of the atmosphere, and preemptively spread our parachute out behind us ready to catch the first whisps of air. Why can't the parachute just deliver us safely back to terra firma?
Because power = force times velocity (yes, TAS again).
We might not have much drag, but if we are doing 8 Km.s-1 we are going to burn up. (i.e. too much collision energy to transfer to something else without getting all crispy). Our cloth parachute isn't up to the job, so we need a better one - an ablative heat shield will do quite nicely. That is just a more robust drag producer after all.