The article says that the grenade's travel distance is measured by counting its rotations, implying that the barrel is, in fact, rifled. (Maybe that doesn't technically make it a rifle, but it doesn't seem like a terribly inaccurate term.)
Technically true; however, in practice, you must rifle the barrel of any grenade launcher, or else have tail fins on the grenades to prevent aerodynamic tumbling. Tank guns, cannons, howitzers, and the like typically have rifled barrels but we don't call them rifles. (Mortars, on the other hand, use finned ordnance.) AFAIK rifles are usually defined to have a caliber at most 20mm.
The British have been trying to replace that rifled gun with the German L/55 smoothbore, too, but they don't have the money. It wasn't clear at the time the tank was designed, but the rifled barrel turns out to have been a mistake. The round for which it was primarily designed (High Explosive Squash Head, or HESH) hit a sort of technical dead end with the introduction of spall liners and layered composites in tank armor.
Read the comment by Steven Den Beste, who explains the situation pretty well (aside from getting the HESH acronym wrong):
True, it is grenade launcher. One thing that makes it more rifle like is the trajectory, it's much flatter than a 40mm grenade launcher. M203 and M320 grenade launchers launch long range grenades in a high arch, like a long football pass.
