Harry can be very smart despite what the movies want you to believe.

Besides if is was about brains then Hermione could have simply told Harry, unless She also has 2 brain cells, which I know she doesn't

If Harry had used the invisibility cloak then his spells would just hit the cloak, unless he raises the cloak to cast spells, in which case they would see him any way and they could then easily summon the cloak.