Ngrams is very hard to use with the word clone, not only is there a lot of "closed" "clothes" "close" and similar misspellings that come up as cloned and clone and cloning, but google apparently thinks the geological survey is a work of fiction, along with zoology and chemistry journals :^/

Hmmm... a quck bit of googling seems to indicate that while cloning does pop up in sci-fi before this, it's never called cloning, Brave New World has de facto cloning as does The World of Null-A (which was serialised by campbell during the middle of the original foundation stories' run), so yeah I have to agree with you that it's probably a later addition to the text, unless asimov was bizarrely enough the first sff writer to call cloning "cloning" (which is unlikely).