Btw your "with additives" "without additives" pictures are a joke, you showed a part of a tank with what looked like hc and a part that may have had no plants and compared it to a tank that obviously has a hair grass carpet...
Those plants behave so differently you can't even compare them.
Yes, they need to consider total biomass, not just root growth alone. Many species have very different root/shoot ratios.
The whole business about long term growth is always a dead ringer.........since few aquarist can not measure measure or determine long term growth differences. Heater cable people tried this same old argument. If there is no difference in the rate of growth, then..........you cannot say much, if it is a subtle thing etc, over long time spans, then by that very definition, it is not a significant factor. This is circle talk marketing.
I consistently sell a lot of wimpy hard to grow plants out of several tanks, the roots look pretty healthy. I've seen AFA's tanks and they have BGA below the glass also on some tanks with all the ADA fixings.
Maybe the CO2 is different, maybe the plant species are different.
This may suggest something to consider further, but that is about it.
If the CO2 is different then the plants from the CO2 low tank will add less O2 to the sediment. More growth= more O2 to the roots.
One reason the cable and the additive folks bother me is a simple fact they do not address the plant's own abilities: they add O2 and everything else they need, the bacteria will establish quite well on their own. That is taking a natural
instead of a chemical
approach. Maybe some like chemical approaches, I prefer to keep it much simpler.