I don't think I agree with this:
I think if we look to AD&D 2nd Edition, we can see Zeb Cook getting more explicit about what was already implicit in earlier editions of D&D (i.e, why did Clerics have level names drawn from real-world religions in 1e, why are 1e Clerics "forbidden to use edged and/or pointed weapons which shed blood," etc.). Hence why Cook mentions the Crusader military orders (who show up a lot in fiction), Archbishop Turpin of the Song of Roland, and the Sohei (and I would argue the Yamabushi as well) as inspirations.
And if you're looking for fictional clerics who trend more towards the divine spellcaster archetype instead of the holy warrior archetype, look no further than the reams and reams of medieval hagiography that depicted saints (of various levels of historical authenticity) being able to call on their God to heal the sick, bring people back from the dead, smite the unbelievers, banish demons and spirits, and so forth.
So I think there were already fictional clerics out there in the cultural zeitgeist when D&D was being developed.