As I mentioned in my previous post, flowable concrete (not to be confused with cement, which is a binder used in the concrete mix) can have too much water. However, additives can make a low-slump concrete more flowable without too much water. As long as the concrete is properly vibrated once placed, separation of aggregate (usually gravel and sand) shouldn’t be a problem.
Your concern is well-founded; I have seen formwork removed and all the aggregate was sitting at the bottom - there were huge voids in the concrete surface, which meant there was no aggregate in the top of the wall. The consequence of this kind of wall means that there is no material, like gravel, “locking” around the reinforcing bar in the wall. The concrete at the top of such a wall is more likely to crack, allowing moisture in to oxidize (rusting) the re-bar. This in turn leads to spalling of the surface, allowing the smoother, denser exterior finish to expose a more absorbent core, which leads to the ultimate demise of the wall.