Well, he had a specific name for it, so it's likely some unique technique he's developed rather than just willpower (Itachi doesn't seem to need to make hand seals like other ninjas). Considering there used to be more Sharingan users, I'd think other people in Konoha would know if one of the abilities of the eye was to spit out fire, nor does it fit with the eye's other abilities of observation and illusion. Too direct.