A bit too reductive, methinks. Even if you're only speaking in the literature professor sense, the vocabulary and sentence structures are a bit more varied and exercised than in a "very ordinary" style, words I'd reserve for mainstream best-sellers like Harry Potter and other YA fodder.
I'll be honest: my (for now quite small) experience with the /lit/ canon has been very hit-or-miss for now. It feels like Literature with a capital L is something only ivory tower academics and New-Yorker or Parisian critics have the right to stick to works and they decided that comedy cannot be part of it. They're also very quick to ignore small and larger crossings into purple prose territory when it pleases them.
Similar in a sense to the music world, where the crowd that decides what is Serious Work^tm is enshrining contemporary classical, post-modernism and performance art instead of sifting through the (admittedly lackluster) popular music world that has produced genuine pearls often rivaling, sometimes eclipsing their dominion.
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End of rant, sincerly sorry.
tl;dr: yes, Pratchett isn't literature in any sense of the word (nobody claimed so, anyway) but your scale may be a bit too logarithmic, putting him and much worse stuff in the same basket.