That hasn't been the case for me but my 0 probably isn't true 0 but test results are all I have to go by.
I think you're right that "test 0" is not the same as "true 0". I was waking up every day in my near-zero system to scrub and remove the fistfuls of GHA that had regrown overnight, so it's definitely pulling nutrients from somewhere.
If GHA *does* require phosphate, and is able to pull enough to completely regrow each night, then IMHO one of the best ways to remove phosphate from my system is to manually remove as much GHA as I can every day, thus forcing the GHA to pull even more phosphates, grow, and then get removed the next day.
I really don't like using filter socks, but they helped me put a huge dent in GHA by filtering out everything I scrubbed off the rocks, and by filtering out all the new growth GHA that colonies send into the water column overnight. I may have been running filter socks through the wash every morning, but only because they really do work and there was a lot of GHA clogged in them in the morning.
Whether I could measure it or not, I still credit the GFO with removing the phosphate that was present regardless of what my test said--if phosphate is available to the GHA, then it's available to bind with GFO instead of feeding new GHA.
I may have also had a lot of phosphate bound in my rock structure that doesn't show up on tests but is available to whatever the GHA uses for roots in the rock structure. If so, physically removing the GHA forced it to deplete rock phosphate just to regrow every night.