Issues with insufficient AA will usually show up as noise or "crawling" pixel effects, rather than "flickering", which I would characterize as a lager-scale effect. If you're really seeing "flicker", it's probably either a GI issue, or some kind of geometry shadow casting issue (out of frame geometry being calculated at too low a detail for example).
From what you describe, where you see flicker in displacement-based simulated "vegetation", I think there are 2 likely possibilities. Either your primary rendering detail is too low, causing geometry calculation differences between frames, or there may be spiky displacement that is crossing render tile boundaries and not getting rendered fully on some frames (which would be fixed by increasing Displacement Tolerance in the planet node). It would be really helpful to see a brief clip of this "flickering" to really be sure what is causing it.