The reason some people say its retconned is because thats the appraisal of that situation as far as the Official Handbooks are concerned. Unless you are already aware of this and have some new information of knowledge that may possible override that fact? Chronologically its all wonky and even more so is the story - does that mean Death in that story wasn't Death? Or Rot wasn't Rot? Or some other weird combination. Still. Hope that helps and that Ka-zarr story was really bad. lol
Well the Rot was a creature born into existence the moment the Original Thanos died against Adam Warlock, and Death explained that for an instant her energies and those of Thanos had unified briefly when he died. In that moment the Rot was created. Minute and undetectable to all. It was alive because of Thanos, but Not-Alive because there is no life in death. By the time any of the Cosmic Hierarchy noticed it, it was already massive in size, and there wasn't anything they could do about it.
Honestly, there was nothing wrong with the story or its placement. The only reason I can even figure why they would try and cut it out is because they wanted to retcon Mantis from her Madonna status and de-power her for when she gets stuck in Annihilation, Guardians of the Galaxy and Imperative. In Celestial Quest she went from being powerful enough that Thanos could not "easily kill her", and then gets dropped down several tiers for later arcs.
Quoi left to explore the universe with his pirate-lizard girlfriend and that was like the last we see of him. I don't think he's ever appeared since then.