TOED specifically says of the 'flocci...' word - and I quote - "humorous...enumerated in a well-known rule of the Eton Latin Grammar..." (The underlining is mine, of course.) It then goes on to quote from Shenstone et al, as you say, Kempie.
Given that it was, according to the 'bible' of English etymology, undoubtedly created with humorous intent, in what way is it any different from 'supercalifragilisticexpialodocious' (34 letters)? Or, for that matter, from my own quite seriously suggested 'antidisestablishmentarianistically' (also 34 letters)?
I cannot think of a single reason why the Guinness Book of Records of all publications should be taken as an authority on what constitutes a real word. My adverb, which I created a few years ago, is just as much a real word as the 'flocci...' one or the 'supercali...' one and it is certainly longer than the former.
If actually seeing these words in print is not a test, then surely the 'pneumono...' one - also, as you yourself point out, a word deliberately created by a puzzle-maker - knocks the 'floccinauci...' one into a cocked hat...not to mention my earlier-mentioned 'methionyl+1900 letters' one!
And there I'll let the matter rest, leaving you, Potatoman, to make up your mind as to the longest word in English. Do bear in mind, though, that 'longest' in these circumstances is virtually meaningless.