I don't really see how it can be described as anything other than a "home made approach" given that they descended out of whatever FL at which they had been cruising, and tried to fly what they thought was a straight in approach to an airport with no IAP in at best very marginal VMC. After all: if they really could see the airport, how did they come to be victims of CFIT?
Although I have no idea if this is true: a sceptic might conclude that when the controller asked if they could see the airport, they might have said yes in order to avoid getting into trouble because flight in IMC is not permitted in Germany unless on an IFR plan.
I agree that, had they followed the procedure you described, they might well have made it. Equally, that may have been what they were trying to do and they just got it wrong. Either way, they didn't get the indications that things were wrong that they would have had on a properly coded IAP.
That wasn't really the point I was trying to make, though, which was in reply to an earlier post: it was simply that there isn't always a black and white distinction between flights under IFR in airways and flights conducted completely in class G between non-instrument airfields.