Why does everyone assume that Biscuit knew Mr Fingers?

Sure they were in the same dungeon, but it didn't seem like the demon that had him bound was letting him out of its sight much.

And maybe, just maybe he's not quite as wise as he likes to believe?
By the rules the bonus to mental stats stops at +3, no matter how old you really get. I know Thunt doesn't always follow the rules, but there it is. And Orcs and Fighters aren't often known for their phenomenal wisdom to start with.
So even a "wise orc" might have less wisdom than your average human.