With the 'right' mix you can use up to about 40% EWC.
Most studies looking at usage rates of ewc find from 10-20% is ideal, over 20% isn't needed and has been found to offer no additional benefit. There has been a lot of study in this area in the past decade. WormPower has provided much ewc for study by various US University.
Adding too much OM with high bulk density will decrease air porosity and increase water-holding capacity too far, which is not good. We want to insure air porosity stays high in media for best growth of plants and aerobic microbes. The pic you posted of the plant in the tiny pot doesn't mean tiny pots are good, they aren't. Right after watering a pot that small there will be too low in air porosity (i.e. anaerobic media) unless the media is 100% peat or coco, etc.
RE: humacarb:
It looks like it's simply micronized lignite or leronadite, even the company claims it has "low solubility". Basically it seems that Humacarb is straight lignite/leronadite, which won't provide the benefits that humic acid liquid will provide. And humic acid liquid will provide many of the same benefits that used to be only attributed to fulvic acid.
Humic acid liquid is not synthetic as Humacarb seem to infer, it's the same stating material as humacarb (apparently), but through the process of exaction and neutralization the humic substances become bio-available and soluble. The humic substances in Humacarb are not bio-available (to the plant) nor are they soluble. Humacarb seems to be best used as a microbial feedstock, ex. to feed fungi.
Humacarb isn't nearly the same thing has humic acid liquid if Humacarb is merely micronized lignite or leronadite. Humacarb will not offer anywhere near the bio-availability to plants as does humic acid liquid.
It's probbly a better be to include humus rich compost or vermicompost to media than to add humacarb. It seem like GH "Dimaond Black" is the same thing as Humacarb...