Actually that is the entire timeline. Maybe you were looking for the dopesheet view? The timeline view can be useful if you have the dopesheet on the right or left of the viewport and you'd like a second timeline below your skeleton, to avoid moving your mouse very far just to scrub the timeline.

Spine uses a pane system for laying out views, so we can't prevent you from making the timeline viewer taller than is really useful. I suppose it may be less confusing to center it vertically rather than have that dark space.