I guess my question is why do these low levels of phosphate work for Acropora in the ocean but not in our aquariums? We have created a more optimal environment for Acropora than wild coral reefs?
The easy answer is that they get phosphorous from other sources like poly/meta phosphate, organically bound phosphates or they can assimilate things and use their phosphates either with polyps or through the slime coat.
I would argue that they can do all of these same things in captivity save for using polyps to catch food (mostly). In captivity the orthophosphate levels might be higher, but some corals just tolerate them... they don't actually use them in lieu of polyphosphates, organically bound phosphorous or catching bacteria in their slime coats.
There are many tanks with very nice corals that have ocean level residual po4 levels yet I doubt that they are phosphorous limited at all.
Remember that we can only test for one kind of phosphorous - orthophoshate. There are many other kinds out there. There is also all kinds of research that ortho is the least desirable form for many things in our tanks to use.
I think that since ortho can be tested for, it gets credit where it is not due for supplying coral with phosphorous. IMO, it is better categorized as a waste product that could indicate that if you have a trace you might not be phosphorous starved further up the food chain, but it might not mean this at all...