Good to hear. When I added GFO to a tank of LPS & zoanthids about 4 years ago, I remember seeing some of the LPS like Acans shrink up, lose some color & didn't extend their tentacles for a few days but they adapted & everything returned to normal fairly quickly. I did not attempt to phase in the GFO - i start d with a large amount 24/7 as I was in the middle of a Bryopsis outbreak. My Zoas stopped multiplying as fast as they once did. Nothing died as a result but my Florida Ricordias never regained their former glory. But it was a good trade off for having a tank with algae under control.
You probably still have a very small amount of PO4 in the tank, despite the 0 ppm reading. It's just lower than the meter or test kit can detect. It's probably enough for your coral's metabolic needs. But levels of .01 -.02 ppm, sometimes even higher, are sufficient to suppress most algae and provide nutrients for corals and micro fauna, IME.
Your tank reacted very quickly it seems and it was neat that you did a time lapse video to document it.