The ECU only has an electronic rev limiter - meaning it can hold the rpm by removing fuel and spark.
It however cannot prevent a mechanical rev limiter - meaning if you shift from 4 to 2 at 120 km/h and pop the clutch - engine spins to a few billion rpm and you end up with scrap metal engine. - I see this one has an automatic gearbox, I dont see how one can do a mechanical overrev without trashing the gearbox.
That sort of valve failure you see in the picture happens from the valve being stressed, this sort of wear is accelerated by bad valve clearances which overheats the valve badly (very badly on a turbo engine). Such heat also damages the guide and lets the valve sit askew on the seat (which is poor anyway because of tight clearance). Repeat this a few billion times and you essentially are bending the valve poppet in different directions on every work cycle. - The end result the valve head lets go from where its welded to the stem (congratulations, your valves and pistons made tiny baby pieces of valve and pistons)
I would scrap that cylinder head - it will create bad spark knock in that cylinder, the piston too. By the colour of the engine varnish I think the owner was using low quality dino oil in there (bad juju for turbocharged engine)