As Jeremy said, the KS list is a little loose with standards. I try to be as as inclusive as possible unless I feel like there's a good reason NOT to accept it.
The high score list is the stricter one, that needs either 1) evidence, or 2) verification outside of DKF.
In the case of Wiebe, he has always said that he achieved numerous killscreens and scores "in the 900s" during his college days in either 89 or 1990. I don't see any reason not to believe him.
Here is an interview with Billy and Steve from Classic Game Expo 2004 where Steve talks about DK in his college days, the relevant part starts at about 8 minutes in.
It's definitely worth keeping in mind that if Steve isn't being truthful here, the motive couldn't possibly have anything to do with the King of Kong, because neither Steve nor Billy had any idea at the time of this interview that a film was going to be made. KoK didn't go into production until the following year.
At one point there was strong reason to believe that a particular claimed KS didn't happen (there was no evidence, and strong circumstantial counter-evidence), but that situation only came up once.
