Kind of like ‘magic user’, there are different kinds of druid. Mages can have different sources - Sunwell, nethermancy (Draws directly from the twisting nether - not sure if this has been retcon’d), ley lines, etc.
For druids, they can draw from a few places, and there’s a decent amount of mish-mash - for example, we see laser chickens with lunar/Elune-themed spells, but you also can see mushrooms pop up, shape-shifting, and general ‘growth’-themed spells. Druids can draw power from intermediary spirits, such as Loa or wild gods (Cenarius \o/). Kul Tiran druids learned from the Drust who are a whole 'nother thing, and they focus on decay/death-oriented stuff - which, will nasty, is still a part of nature. Shapeshifted forms are, well. Just look at Guardian druid and the Ursoc-themed…everything. And it all, insofar as I know, kinda boils down to the Emerald Dream, which is a version of Azeroth at its ‘base’ - all natural and everything.
So, harvest warlocks/witches are basically more…How to put it, down to earth? They haven’t learned the methods of the night elves, who are more deeply connected to the wild gods and the emerald dream. I’ve mostly seen them described as ‘basically like night elf druids, but less practiced and less connected’ - they also believed in more ‘local’ spirits, such as harvest spirits (hence the name harvest witch/warlock).
As best I can figure, harvest druids focus on smaller, more local wild spirits (such as a spirit for wheat, another for fruit,m another for vines/grapes, etc) and their own traditions - whatever they may be (in this case, probably Gilnean), and they haven’t yet managed deeper connections with stronger wild spirits such as Ursoc and Cenarius, or achieved some sort of connection with/through/by(?) the emerald dream.
TLDR: They’re still druids. It all comes from the same place, technically, just the ‘place’ is absolutely huge and encompasses a great many entities and locations, and folks have different levels of experience with and connection to these areas/entities.
For my roleplay, I’ve written it more as a focus on the land itself - my character ends up weaker in cities, for example. I don’t really like this description but ‘night elf druids but not as experienced’ is, based on what I know, an apt description - so they can still heal. ‘Heal in the way of harvest witches’ basically means ‘druid healing’ + whatever local traditions, spirits, and medical practices were available.