Did a little research on Zamperla's website, the designers of Surf Dog. Their Disko models aren't considered coasters by their own designer.

http://www.zamperla.com/en/zamperla-products-rides-family_rides

But Surf Dog is a Disk'o Coaster.

http://www.zamperla.com/en/zamperla-details-rides-disko_coaster

^But why is it listed under coasters and thrill rides while the coasters are specifically under coasters and none of the other disko models are under coasters? RCDB doesn't consider them coasters either.

It looks like Zamperla considers it both a coaster and a thrill ride, and that the hill in the middle of the ride is the difference between the Disk'O Coaster and all the other Disk'O type rides.

(Edit to add) Rcdb.com is just a listing of what the guy running that site thinks are coasters. He considers Blazing Fury (Dollywood) and Underground (Adventureland) coasters, but no mention of the gravity pretzel rides. Other people have differing opinions on what a coaster is.

Is superman the escape @ MM a coaster? Is wicked twister a coaster?

Superman: The Escape at Six Flags Magic Mountain is history.

If either of the two rides you mention are\were not coasters, neither was Demon at Kings Island. Nor Cedar Point's first woodie.

How anyone can claim Haunted House at Camden Park and Devil's Den at Conneaut Lake Park are not coasters ( while saying Underground at Adventureland in Iowa is) is beyond me. I know Duane. He's a great guy. He's done a fantastic thing and kept it going for years. In this case, though, I think his determinations as to what is and is not a coaster are inconsistent.