They surely would have factored those forces into their calculations; and they act uniformly in all directions. What this suggests is that there some force which is not uniform acting in opposition to the horizontal expansion centrifugal force tends to produce.
My bet is on magnetism. Sunspots evidence a high level of magnetism - and they occur near the solar equator, not at the poles. It's theorized that this magnetism hinders the upwelling of hot gas from the core, making the sunspots cooler than the surrounding surface (which is why they appear darker). I would guess this also shrinks the solar equatorial region, counteracting centrifugal force.
It sounds good, anyway.