A 90' turn is an instantaneous change in vector. The Harrier doesn't do that. A Harrier still has momentum. It can make a sharper turn than a conventional aircraft, but not the "ping pong ball in a glass jar" direction changes they've tracked these objects doing on radar.
I agree that your narrow straw argument definition is interesting but it shows that you don’t understand what’s going on when you’re trying to get on the Harrier’s tail. He pulls a VIFFing (vectoring in forward flight) maneuver and it APPEARS to have cut a square corner.
It’s very astonishing to the chase pilots. You aren’t even flying behind a Harrier and you are STILL bewildered.
Upgrade that vectored thrust engine with stronger, more advanced higher G-force propulsion for drones, ya got that ping pong ball effect. Upgrade it still further, you got minimal IR plumes.