Indeed. But one can't have it both ways. You either use your definition and call out ignorance/mistaken explicitly or you define lying as covering all unthruths.

Fulton Sheen cheats by pretending to use your definition but subsuming more than deliberately lying under the liar banner. Very sneak these philosophers.

You may be operating on a deeper level than me here, but it seems like that Fulton Sheen quote just claims that being mistaken or ignorant about whether or not you are God makes you "a lunatic". Whether you use that terminology or just replace it with "incorrect about stuff", it still seems distinct from being "a liar".

(I think we're in agreement that being mistaken about things is not incompatible with being a "great teacher", as I say in another comment here.)