You're incorrect in that this product is not intended for soil.
IMO, that's like saying worm castings (or urea) are intended for hydro. Just because a label says it
can be done doesn't mean it's ideal (it would create a mess). :)
The way I look at it: Hydro nutrients are readily available to the roots. If used in soil, the soil only plays the role of medium (not a living organism, no more than the a reservoir is a living organism -- which hydro growers go to
great lengths to prevent, dumping reservoirs every 2 weeks; chilling to prevent slime; using peroxide to sterilize; etc.).
If a product is going to be poured into soil, why wouldn't a soil product be poured and
encourage soil activity? Why pour something intended for a relatively sterile environment, not needing soil, to directly feed the roots, bypassing the dynamics of the rhizozone? Presumably soil was chosen as the medium for a reason (instead of soillless peat or coco).
Soil tends to be denser/heavier than soilless. Harder to flush (which soilless growers do a lot). Soil is typically watered less frequently. Without the roots and soil working together to make nutrients available, the plant gets everything from the immediately-available hydro nutrients poured in. If it takes 4-5 days to supply fresh hydro nutrients (immediately available), that's different than the typical 2-3 day soilless feeding frequency. (You have to feed stronger less frequently to get the same nutrients to the plant.). That sounds different than roots interacting with microbes thriving in organic matter, receiving the nutrients the plant needs during those 4-5 days (organically).
I've used hydro nutrients in soilless. It worked fantastic. I can't speak for the millions of people whom you say use it in soil. (wink) But, it sounds *far* from ideal.
BTW: I don't think this is an either/or topic. I'm not religious about organic. I try to do more (organic sources of nutrients), not less. (Put another way: I try to avoid inorganic compounds when an organic source of nutrient is available, relatively fast acting. But, I'll use inorganic products if it's more convenient.). To me it's a matter of feeding the soil (not just the plant). I don't understand choosing soil as the medium and then using fertiizer that bypasses the soil (a missed opportunity to feed the soil, and promote that balance between roots, microbes and materials).