Strange thing, it is not an oldtimer. To get yellow plates the car has to be at least 25 years old and be in perfect condition (75% orginal parts). However some people manage to bypass this law (I don't even know how) and register younger cars as oldtimers. I have already seen here one younger car registered as an oldtimer.
The bypass you wrote about can be used if the car is somehow unique - a rare model, or rare series, had a famous owner etc. In this situation - in my opinion - the owner just wanted to "look cool" on the streets.