Just for the sake of interest, here's what the link I was looking at says about the introduction of KOR. I missed a few things skimming too quickly before. (This appears to be coming from an online text of the New Zealand Dictionary of National Biography.)
"Kidd's first major success came from a cross-pollination of Delicious with Cox's Orange Pippin in 1912. Once the plant fruited it became obvious that it had commercial potential, and Kidd planted five acres in the new variety, which he named Delco. He sold the propagation rights to the New Plymouth nursery firm of Duncan and Davies for £2,000 in the early 1930s; they marketed it as Kidd's Orange Red."
If this is correct, then perhaps the date cited by seedsavers would be when Delco/soon to be KOR was introduced to market? But if the original cross-pollination of Cox and RD took place in 1912, then I'm thinking that the strain of RD used would have to date back somewhat earlier than that.
That being said, the DNB also says that "Although impressed by the attractive appearance of new American varieties such as Delicious and Jonathon, [Kidd] was less satisfied with their flavour. Seeking to combine their visual appeal with the better flavour of the familiar English cultivars, he commenced a breeding program." So maybe even Kidd's Delicious wasn't as delicious as all that, at least to Kidd's taste. Or maybe he was canny enough to emphasize the Englishness of his apples to their initially NZ market. Who knows.
In any case, it looks like there may be a fair bit of aphid squishing between me and my first taste of Kidd's...
Incidentally, and surprisingly to me, Wikipedia dates the introduction of Wonder Bread to 1921.