There are areas all over the Earth where rivers and streams flowed along seams of fractured bedrock carving out valleys in the process. If the bedrock happened to be riddled with square block jointing, square forms could be the result. This might well be the case in that Mars example. On Phobos you don’t see these etched out square block features because there was never any water flowing there. But this doesn’t mean the underlining bedrock, at least in places, isn’t square block jointed. Most of it might be covered by the current surface.
If that’s what ya want to believe be my guest!