But a planet (if it’s in our solar system!) covers a bigger angle. When the light from Olympus Mons is deflected away from you, the light from Valles Marineris is being deflected toward you, so it pretty nearly evens out.

To twinkle, you have to rapidly see more light and less light from the thing. Little fluctuations in the atmosphere do that. If the light is from a source of very very small angular size (like any star bar one) those fluctuations divert the light away from you for a moment.