NPCs not being able to interdict T9s comes to design on NPCs being too lenient in general, and never properly scaled up to be a threat to engineered ships.
No engineering can change the way a ship behaves in supercruise, this is nothing to do with engineering. If it's not physically possible to escape an NPC interdiction in a trading ship, then the interdiction minigame would not be fit for purpose (it is a false indicator of possibility) and it would feel like the NPC was cheating (because it is a computer not bound by having to make physical inputs). That's why the bar for NPC interdictions has been set at a threshold such that it is possible to be consistently successful in a T9. There's no other way to do it which isn't player-hostile game design.
And almost no ship can escape player interdictions properly because the minigame doesn't work well enough in an opposed format, you can be considerably more agile in sc than the opponent and consistently in the blue zone
and still be losing because the other guy is winning on his screen and it favours the attacker (latency probably also sticks its oar in). That's why "submit and high wake" is the way to evade no matter what you fly.
Powerplay needs to get to a point where the inability of a T9 to be interdicted by NPCs
no longer matters because the interactions with them are moved out of supercruise and into realspace where they can be spawned in such a way that the player has to do
something about them (fight them, have enough beef to bully through, go dark and try to sneak in, whatever). This would, as a bonus, give players looking to intercept haulers a defined area of realspace to also look in, and make sensors vs. stealth relevant in any part of the game at all if it was also large enough.