Low phosphate as a problem in a reef tank is pretty much unheard of. Hobbiest test kits (and even hanna checkers) are only so accurate and only go so low. Phosphate needs to be incredibly low to cause a problem, and we are contstantly importing it via food.
Phosphate is good at becoming a limiting factor for most nusiance algae when it's down to about .015 or less. .015 will test as 0 on most test kits.
I suspect the thinking that algae needs phosphate to grow in a refugim is more extrapolated from the thinking behind carbon dosing than anything we are likely to encounter in the hobby. (nitrate can/may become a limiting factor with carbon dosing, hence the common practice of running GFO with carbon dosing to keep the phosphate down. While in theory it could work the other way around with phoshpate becoming the limiting factor, in reality it is very very unlikely that would ever happen.
Also on another note, when talking about tap water in reef tanks of course importing phosphate and or nitrate can cause headaches, we shouldn't forget to consider some of the other stuff that RODI removes such as heavy metals and other toxins that we don't normally monitor, and often don't have the abiliity to monitor, in our tanks. Whatever comes in with that tap water is going to get concentrated over time (as water evaporates and we replace it with top off water) so that what may be harmless now, might cause unexplainable and potentially very significant problems in a year or two when it has built up to a harmful level.