I think we basically agree
The fact that P precipitates Iron, there is no question about it. This has been proven when adding various elements together in one highly concentrated mix. Can't do that.
But, like Steve suggested, someone took this a step further and suggested to alternate days of dosing. And I don't think that is really necessary IF YOU ARE DOSING WEAK SOLUTIONS of fertilizers into the large tank volume.
If you add
one weeks worth of P and
one weeks worth of Fe side by side, at the same time, sure you'll see a white shower.
If you add
one days worth of P, dissolved in quite a bit of water, and
one days worth of Fe, also dissolved in quite a bit of water (as it happens in this autodosing setup) I think the concentrations are weak enough to prevent any sort of precipitation.
I guess this is up to you (the reader) how to do this, just like water changes, lighting a tank, or feeding your fish... everyone does it a little differently.