Think you must be looking at the full 3CD set, and not the Record Store Day release
The 3CD set is part of the remaster deluxe editions of the Cure’s back catalogue, which has been happening in fits and starts for ten years or more now. Last one was Disintegration in 2010. The one you’ve seen is the Mixed Up remaster (disc 1); with rarities including Mixed Up b-sides and original 12" mixes of singles from the 80s (disc 2); and Torn Down (disc 3), which is a new set of remixes done by Smith, and only one of these (Never Enough) is a mix of a song that was on Mixed Up.
The RSD release was Torn Down on its own, though I think the Mixed Up remaster was also available separately on RSD.
So, insofar as remastered Deluxe Editions are always a kind of cash grab, this is one too, but not really any more cynical than that.
FWIW, I quite like a lot of these mixes. They’re not as out there as some of the Mixed Up remixes, but Smith has done a decent job of bringing out another dimension to each song. Helps that he’s chosen a lot of my favourite album tracks (Three Imaginary Boys, A Strange Day, Like Cockatoos). I like the remixes of earlier songs more than the later ones, but that undoubtedly reflects the superior quality of the early material over the later stuff.