Didn't make that claim or didn't intend for it to be taken that way but, I can find nothing that says that the "agility" rating applies when the ship is in SC..This is entirely inaccurate, and so is much of your other conclusions based on it.
I'm well aware that thrusters don't work in supercruise, but different ships have different levels of agility, based on a lot more than acceleration.
All ships do cap out at 2001c though.
But if you're going to claim that, for a given speed in supercruise, a T-9 is just as agile as a Sidewinder, you're simply wrong.
I have though done the test using a sidewinder and a DBX. That was part of why I bought a second subscription.
I have put the ships as close as possible to each other in SC at the same speed and then simultaneously activated a continuous YAW and both ships appeared to turn at the same rate. Same for Pitch.
The Sidewinder clearly accelerated away from a star faster. To me this would make sense since the sidewinder is 25 ton hull and the DBX is 260 tons hull. More mass so gravity has more effect.
It is tricky to get two ships moving close together at the same speed in SC and if they are not close together then they are influenced by gravity wells and that distorts the movements by causing speed differences.
This is part of why I am pushing to have the Billion CR. So I can get a T-9 or 10 and an Eagle and run the same test. Plan on going to a single star system and getting as far as possible from the star and then run the test and record it.
One of those thing on my to do list during the next 30 years.
YMMV