Peter - these are not klukflasker, just decanters with dented / pressed-in sides.
I can not see why not, all bottles with stoppers are just decanters the danish "kluk kluk" is just a reference to the sound the bottle makes when poring from it and this decanter do have the shape to make the glug glug sound.
It is not part of Holmegaard marketing glug glug decanter range, but that do not exclude other decanters from being "kluk kluk" thus it is just a name for a certain sound from the bottle.
I would also describe decanters as "Bjæsk", "Gourgelette", "Kluk-kubus", "Fanø", "Mandø", "Venø" as glug glug decanter although they do not have the exact shape as the Holmegaard tradename "Kluk kluk karaffler" Glug Glug decanters.
I am sure that there is many more examples on decanter that over time has been molded as a variation over the traditional Glug Glug decanter.
To me it is a decanter "Karaffel" but it also have sucked in sides and will give the glug glug sound as all decanters do with this shape.