P is used by the bacteria in the system. The bacteria uptake P and then the coral uptake the bacteria.Not going to matter if the salinity is 1.020 and @Sabellafella was right as the reason why calcium and magnesium were so low. This is more paramount than anythjing else, IMO.
I would not keep carbonate at 12 for any reason, but you have to get well below .01 for burnt tips to happen. There are more sources of phosphorus in a reef tank than just phosphate. Phosphate is just what is left over after organically bound phosphate, metaphosphate, etc. have broken down. Since it appears that stony corals don't prefer to use po4 at all, having some just means that you had some left over.
Coral do use P by the uptake of bacteria and ammonium.
Lou Ekus says the coral need phosphate at 0.02 to start showing a net phosphate uptake.
I do agree he should check and calibrate all his devices but to say po4 at 0.01 is fine is walking a very fine line. 0.01, on a reef yes, they have Infinite resources. In the reef tank, minimum 0.03, IMO.
From Lou Ekus seminar: