The allegory is the description, not the subject.

Exactly - That is why I cannot see why an allegory cannot contain historic events. The two are not related. Although the allegory may be made up - Which under the OED definition it may not always be - The story can contain real people. Therefore the false logic is -

a) a made-up story (therefore not real), or
b) a real bloke (therefore not a made-up story)

An allegory can be a made up story about a real bloke or even a real event used to describe or explain something else. I am sure that the bible stories are very allegorical - But that does not preclude them from containing some element of truth. Anyone who says they are either completely true or completely false can never realy prove that.