Well, as pH goes up, fog and contrast go up as does development rate. As pH goes down, these things also go down. Bicarb lowers pH.
Bicarbonate is NOT a classic antifoggant.
Antifoggants fall into 2 classes, organic (benzotriazole, PMT and etc) and inorganic (NaBr). They all work by about the same mechanism, that is, they adsorb to the grain surface thus reducing the exposed silver on the grain surface, and/or they reduce the available silver ion concentration in the developer. Simply said, they can form salts with silver!
All of these effects are stronger on Chloride emulsions, less so on Cl/Br emulsions, and even less so on Br/I emulsions.