Sounds a bit converse doesn't it, that the zooxanthallae that feed the coral - proliferating would decrease it's growth, but they do. (You can see it, as corals go brown(er) with high nitrate and phosphate and the most colourful corals are those with low nitrate and phosphate (so low-ish zoox) but very bright lighting to compensate, which in turn causes the coral's protective colour pigments to strengthen).

Nitrate is often associated with algae, and indeed the growth of algae is often spurred by excess nutrients, including nitrate. Other potential aquarium pests, such as dinoflagellates, are also spurred by excess nitrate and other nutrients. Nitrate itself is not particularly toxic at the levels usually found in aquaria, at least as is so far known in the scientific literature. Nevertheless, elevated nitrate levels can excessively spur the growth of zooxanthellae, which in turn can actually decrease the growth rate of their host coral.

See paragraph about half way down about Nitrate. It doesn't seem to elaborate on exactly why, and I'm not sure either, but would like to know...