My first suggestion would be to contact the manufacturer as to the advisability of that location for the combo intake/exhaust. Secondly, I would guess that most inspectors would be concerned about waste gases possibly including carbon monoxide. With a direct vent gas FP, you have controlled combustion which, in theory, releases pretty clean flue gases. In the unlikely event that there is CO produced, it is in open air and will convert to harmless carbon dioxide.

My real concern would be water vapor. That is a byproduct of gas combustion and it may condense on the bottom of the deck. Treated lumber or not, that's not beneficial. However, given the relatively few hours that most fireplaces like this are actually used, it may not be a problem. Used a lot, however, there may be problems such as corroded deck fasteners. This is especially a problem with newer treated lumber used with the wrong type of hardware/fasteners to begin with.