But FA can't prove that he is lying if he would have said that, unless the FA can prove what everybody thinks.
I doubt that they have to prove "beyond all reasonable" doubt in these cases. It would've been patently obvious to everyone that it was bullshit if Sagna claimed it was a reference to Burnley's support. Saying that it was a typo would've been a more sensible approach, although he'd probably still have been charged.
Not the wisest thing to do, just hope he gets a fine rather than a ban. That's what usually happens when managers do it.