Looks like the specs are misstated--e.g.,
Clamp Diameter: 25.4mmClamp Diameter: 25.4mmSteer Tube Clamp Diameter: 1-1/8"Steerer Tube: 1-1/8" ThreadlessStem Color: BlackStem Length: 120mmStem Rise: 20 degWeight: 277g \
But, 25.4 mm is a 1" stem. They seem to be talking about the steerer tube in the ad, not the handlebars. If you've got 1-1/8" handlebars you're probably good to go. Otherwise, you'd need a shim.