It's one thing to lie, it's another thing to lie about something that could easily be found out and possibly get them sued. The risk/reward ratio seems way too high for Facebook to lie (unless they were forced to.) That's obviously not proof, but it seems like pretty strong evidence.
That also doesn't count as proof; it carries barely more weight than the Facebook exec's statement.
However, you're quite right about the decompiling argument. And chances are that security researchers have done just that.