If you watch the videos of Marbozir, you can see, that the new districts all get the +6 yields, even if they are adjacent to plantations, camps and other districts. I think at least the building of a district should destroy the jungle under it, so the bonus wouldnt be that high (I think the adjacency bonus of a nearby district is around 0.5 or less it seems). And compared to Civ 5, there you would have destroyed the jungle when you build a plantation. Building a camp will probably still maintain the forest/jungle.