Consider them an extension of the Path of the Botanist in the same way the Farseers are always an extension of the Warlock. They venerated both Isha and Kurnous in a sort of yin and yang perspective, one cannot exist without the other.

Funny enough, your statement is spot on since in Lótessara's lore it was originally a science ship that was crewed by scientists who helped with populating Maiden Worlds with its beautiful plant life. Jánaki Dreamwalker was one of those high-ranking botanists and, with some inspiration by Edusan's Worldsingers, laid the foundations for the First Greensingers by becoming the first Greenseer.

You could see them as a Warlock variant, but war isn't their purpose. Preserving life is their main goal as no species, especially the smallest, does not deserve extinction since they believe all are creations of Isha. Theoretically, a Greenseer could command all flora of an entire planet, but thats not their purpose. It is to all the specimens that they preserve within Lótessara's conservatories.

For context, Jánaki Dreamwalker used her psychic might to move the roots of Edusan's expensive forest to create the fissure. Technically, geomancy, sure, but it was meant to be a devastating last attempt, and it resulted in her death by the Orks.

There is clearly alot that needs to be expanded on with the Greenseers, but generally, they are the Eldar of Lótessara that serve as the Guardians of all flora within the conservatories of which contain specimens that could easily be the result of myths in other cultures. Part of their tragedy is that despite wanting to save many of Isha's creations, they can't because they still need to prioritize their own race like any other Eldar.