Some people find that when they use iron test kits, they get a low reading right away, but if they leave it a few hours, the solution gives a much higher reading. My guess is that the test reagent is grabbing the iron ions whenever they are released by the chelator, so the immediate reading reflects the level of dissociated iron at equilibrium, but as time passes and the reagents grab more and more of the iron before it can recombine with the chelator, the reading reflects total iron. Presumably the plants take up ferrous ion from the constantly available minute amounts that are not chelated, although I think I've read that land plants, at least, may secrete chelators from their roots and reabsorb the chelated minerals as such. The chelating agent protects the ferrous ion from being oxidized to ferric ion. It's this oxidation that is referred to when ferrous ion is described as being unstable in water.
Plants use ferrous iron.
I hope this helps. The distinction I'm referring to is between nitrate vs nitrogen and phosphate vs phosphorous. Flourish does not contain nitrate or phosphate per se, but it does supply a nitrogen source, the Nitrogen equivalent being 400 ppm in the bottle. While I can't comment on unannounced products, I can say that you can expect to see some products in the near future that will complement the current products and give the hobbyist more control over specific water parameters. Obviously the plants need a source of nitrogen.