Infernity Guardian vs. Book of Moon

IG can't be destroyed as long as the controller has no cards in their hand and it's face up on the field. What if you use Book of Moon and then attack it.. can it be destroyed or is it considered face-up again?

I'm sure I already know the answer but it was being debated so figured I'd post it here for the definite answer.

Can't be destroyed.

When you attack a face down monster, it's flipped up on the 2nd sub-step of the Damage Step. That is why Infernity Guardian would not be destroyed in the scenario you propose.

That's exactly what I said. Thanks for proving it.