Thanks for the link to the interesting review. My take is that...

Thanks for the link to the interesting review. My take is that the authors are scientists and want to understand the process involved in the action of biostimulants, and want to sort the effective from the ineffective, and then be able to use this knowledge to inform legislation and decision making around the biostimulant industry. This is a good review and I think the writers are clearly trying to make a case that more research and funding is needed for the research (fair enough that is their job).
In terms of your cited paragraph- it is completely out of context - for a start, it is not their conclusion - it is at the beginning of a paragraph summing up the pros and cons of biostimulants and the state of play, and your quote comes from their cons examples Following the cons, they write:
"Nevertheless, there are a significant number of rigorous independent reports of benefits from some biostimulant formulations and market growth data demonstrates that there is a good deal of support for these products within agricultural producer communities. That such market growth has occured, even in the absence of a known “mechanism of function” suggests that there are aspects of plant metabolism and productivity constraints that are not understood but are potentially important if we are to achieve the goal of increased global food production."
From following para:
"The market euphoria that is taking place in the biostimulant industry recognizes these unknowns and biostimulants are viewed by many innovators and investors as a mechanism to conduct broadscale, if unfocussed, discovery of novel biologically derived molecules. Much as the exploration of marine organisms, and plants and microbes from diverse ecosystems has led to the discovery of novel pharmaceuticals, so too the development of biostimulants from the broad range of source materials, holds significant promise of discovery. Recent years have seen rapid growth in the number of published studies, increased numbers of scientific conferences and development of legal framework and legislation. These trends will inevitably improve the image of this industry and the efficacy of products."

Then they talk about the main problem being we don't know the method of action etc and which ones actually work etc (the call to funding research). The obvious benefit of such research would be that if we understand the mode and action then it might be possible to optimise its effect, and stop shonky operators selling ineffective options.

The actual conclusion of their review reiterates the view of the authors of how important it is to have a scientific understanding of the process so it can be utilised most effectively, and the ineffective ones be weeded out, and the good ones improved. The final sentence of the conclusion:
"Continued investments by commercial entities in biostimulant research and product development will serve as a critical driver of discovery in this realm and will inevitably lead to the identification of novel biological phenomenon, pathways and processes that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate."

The researchers are not dismissive of biostimulants, quite the opposite - they want the interest and funding to make a career out of understanding and characterising biostimulants!
Thanks again for the link to an interesting review which confirms my decision to invest and hold long term in GPP.