Well, I certainly claim no superior knowledge. And I have no technical knowledge of parallels other than to say, “Yep, they’re the same”
I’m also not really knowledgeable in (or quite interested in) the theories of book compilation. But I believe one theory is that the Khp is a kind of a “handbook” text (before there were books???) for novices to memorize. Containing the refuges and the novice precepts make that a rather strong case. So does the fact that it contains one of the few suttas about merit sharing along with the important meditation list of body parts and the three most (currently?) popular suttas, the Mangala sutta, Ratana Sutta, and the Mettā sutta.
But the Mettā sutta is identical in both places. There is a variation between Thai and Sinhala versions, but at least within the Sinhala, it is identical in both places. But I’m really not sure how parallel people define a parallel. It would seem that some difference between two things is needed to qualify, otherwise it’s just the same. I don’t think we ever see completely identical suttas in the same book (or in the case of the KN, sub book or whatever you like to call it.) There may be something in the AN… I have a vague recollection of a footnote saying there are two suttas with no apparent difference.
So it’s not the case that we would have to modify any translations to include the Khp suttas to also be in the Snp listing. At least for the English translations, most if not all are long finished, so the issue of updating changes in both places is not present.
I think this would be a case where at the very least having a link in the suttaplex descriptions to the other suttaplex cards is warranted. Since this will be a gateway sutta for many people, it’s not reasonable to expect them to explore the parallels section to find other translations.