Coining? Not exactly

This week's World Wide Words newsletter has this item:

Philip Arnold e-mailed, “I am wondering if we may credit you with coining a new word. In the snippet on Great Wen you use eruditism. I could not locate it in any online dictionary, finding only erudite. Congratulations.” It’s very kind of him, but I can’t take any credit. Though it’s not in any dictionary that I’ve consulted, not even the Oxford English Dictionary, a search found a number of examples — one from the nineteenth century — which use it in the same sense as I did, for an erudite word. It wasn’t an error for erudition.

I don't quite get this - which I run across all the time, by the way. How is it "coining a new word" (in the positive sense) or "that's not a real word!" (in the negative) when you take a perfectly standard bit of English derivational morphology, -ism in this case (in the meaning "feature or trait", and add it to an English stem?

Lots of words "aren't in the dictionary" because the dictionary compilers made a conscious decision not to include derivations that were transparent, or to put it a bit more technically: Such resources often do not include words that are morphologically derived from base forms on the assumption that morphology is regular and that affixes carry unambiguous information both about the part of speech and the meaning of the derived word. (Putting Semantics into WordNet's "Morphosemantic" by Christiane Fellbaum, Anne Osherson, Peter E. Clark).

In other words, if you put "un" onto a word, are you really "coining a word"? Just because "unwhite" isn't in the dictionary, did I just make it up? If you ran across it in a sentence (the dog's teeth weren't yellow but they were unwhite , say, or he didn't really blacken names, but he took pride in how quickly he could unwhiten a reputation) wouldn't you understand it? How about an evaluation which said "Susie could stand to develope some indistractableness" - clear what it means, no?

In plenty of other languages such use of derivational morphology is unremarkable. How did we get so enslaved by our dictionaries that "that's not even a word!" gets attached to transparently meaningful things such as "unwhite" or "eruditism" as much as to things like "shiltzenry" or "wug"?