Right, but that downside isn't something that kills the corals. The more fragile skeletons don't result in lower viability (at least for the duration of that experiment, I suppose)FWIW, your own reference suggests a downside to high phosphate:
Skeletal density reduction may be due to phosphate binding at the calcifying surface and the creation of a porous and structurally weaker calcium carbonate/calcium phosphate skeleton. Increased phosphate concentrations, often characteristic of eutrophic conditions, caused increased coral growth but also a more brittle skeleton. The latter is likely more susceptible to breakage and damage from other destructive forces (e.g., bioerosion) and makes increased coral growth a poor indicator of reef health.