But the convex shape itself is not sorted - the depth sort order is based on the view direction, which is not the same every frame. Since the buffer contents remains the same and the viewpoint changes every frame, then this can't be a valid argument about the ROP processing order since sometimes you would be in the reverse order. Or am I thinking about that wrong?
The view direction doesn't have to change each frame. Pick any one view direction and render the shape from that direction (and optionally vary any other irrelevant details that might influence race conditions, like processing load, other surrounding operations, phase of the moon, etc). You'll get the same result every time regardless of situational details that would influence race conditions. You always see the same triangles drawn over the top of other triangles, with no pixel/block artefacts from threading bugs within triangles. It will appear as if the GPU has rendered the triangles one at a time in the order specified.
In "foliage rendering in pure" they make use of this property in terrain grass rendering, by having a "tile" of 3D grass planes, with 8 index buffers, which each represent the planes as sorted correctly for 8 different viewing directions. When rendering, they pick the index buffer that has the most correct sorting order for the current view direction, which gives them almost correct back-to-front triangle sorting within each batch of grass.