If it did pull in too much air then the worst it would do is spit out air bubbles along with the hot water. Maybe a bit messy if the water splashed a lot when this happened but the opposite problem is more likely and you end up with insufficient air in the tank and the pressure gets too high...
I had one of these installed thinking it was a good idea but unfortunately the venturi device does not draw in sufficient air to keep the air gap replenished. After several months I find the vent pipe outside is dripping when the tank is being heated and I have to shut off the cold inlet, fit a...