This does not mean it is not a posteriori.
"The theory of intelligent design works well in both directions, and it covers every conceivable situation in the known universe."
It therefore is completely worthless. It explains nothing in particular.
"If you think you can engage science without any a priori method then you may indeed be a candidate for belief in the spaghetti monster."
If you think that you can have a scientific theory that can never be falsified or tested, because, as you said, "...it covers every conceivable situation in the known universe.", then you haven't a clue what science is.
It particularly explains why there is, on a universal scale, the presence of organized matter that behaves according to predictable laws. This is valuable because it relieves the observer from anticipating occasions where matter will behave in a way other than the intelligent designer intends. It is also valuable because it instills in the observer a sense of respect and awe for the manner and degree of detailed design with which the observable universe is imbued.