Well it does appear to have nothing to do with actual over-revving after all but, IMO, Gijs is wrong about it not being able to be turned off.
The HDV line is:
"DownshiftBlipThrottle=0.98 // amount of throttle used to blip if controlled by game (instead of player)"
"if controlled by game" means if you have set it to autoblip in your player.json. If by control "of player" = you have it OFF in your json and your foot controls it.
I'm now wondering if the anomaly it has to do with what may be a neutral gear rev limiter. Not hugely common, but some cars have it, although I think it odd to have it in a race tune.
Clio's Engine.ini, line:
RevLimitRange=(7500, 100, 1) I'm would say the first number is the normal rev limit of 7500RPM
RevLimitSpecial=(0,,,"gn=-2500") I'm guessing this may be a neutral gear rev limiter gn= (gear neutral?) -2500 subtracted from 7500 = 5000RPM, about the limit if the throttle is floored while in neutral.
Test: Sit in the garage with clutch out, gearbox in neutral and rev the engine to max throttle. At best you'll get 5000rpm out of it and the upshift lights come on.
I wonder if the lights are simply triggered whenever the revs are above 5000rpm between gears (i.e., in neutral) as you upshift or downshift.