Since we're arguing semantics, it should be made clear that the use of the word Muppet here is correct. Saying the Muppets were involved doesn't limit the reference to only Kermit, Fozzie, Piggy etc.. as defined by the Henson Workshop: "Individually, a Muppet is made by Jim Henson or his company's workshop."
The performers at the Grammys were indeed Muppets by that definition, just as are the Skeksis, Ludo or the LaChoy Dragon from years ago. For that matter, it's got nothing to do with Disney either, at least to a certain extent. Muppets found on Sesame Street are separately owned and copyrighted by Sesame Workshop and The Jim Henson Company which are not part of the Disney family.