There was another article in Billboard that year stating that RCA had abandoned their single-inventory approach pretty quickly (it started with Hugo Montenegro's Godfather, APD1-0001, and ended I think with Jose Feliciano's Compartments, APD1-0141) because retailers were filing the LPs in a dedicated quad section instead of mixing them in with the stereo LPs as RCA had intended.
I think RCA would've had more luck with this approach if they'd had a more subtle sleeve design (maybe like Ovation, which buried mention of the QS encoding on their single-inventory LPs at the bottom of the back cover) instead of the dark black border with the big red and yellow 'quadradisc' logo at the top, which looks like anything but a single-inventory design.
I don't think what RCA was saying about becoming a single-inventory label was just lip service either, I think they were fully pursuing that aim. Since they owned all their own studios and forced their artists to use them (and their producers/engineers) as part of their recording contracts, they could implement this kind of workflow where most other labels couldn't. It was actually the smart/cost-effective way to do things, because they were only doing (and paying for) one mix, the quad mix.
As a result of pursuing this single-inventory approach, I have a feeling that a lot of what RCA put out in stereo in the 1972/1973 era were actually fold-downs from discrete quad mixes, and only came out in stereo when it became clear that this strategy wasn't working. Who knows how many of these tapes survive, but if they do, there should be a bunch of unreleased quad mixes in the RCA vaults.