AI flanks in RTW Vanilla too, that's not the problem. The problem is that cavalry can be micromanaged to isolate and destroy AI units one by one. They have no clue how to cover their own flanks while advancing toward your main army. Those screenshots show the AI sending one unsupported cavalry unit moving to his flank, so it seems this problem hasn't been fixed. Such isolated units are easy prey to human controlled cavalry.
That fact that he loses is because he sits still without proper flank protection and the AI probably has better units.