You are right. For non planing hulls, the hull speed is limited by its waterline length. It has absolutely nothing to do with the wind or what point you are sailing on. Boats are slower to weather because they beat into the seas which slow them down and because they are going more directly into the wind.
Hull speed is a different thing. The speed of a wave is 1.34 times the square root of its length (distance between crests). The boat creates waves as it moves through the water. At 1/3 hull speed there are three waves formed along the windward side. At 1/2 hull speed, the waves decrease to two. At hull speed, the boat creates a wave a little longer than her waterline length and gets trapped between these two crests. Unless she has flat sections aft like planing boats and enough power to climb up on the wave and start planing on her flat sections, she can not exceed this speed. Roughly, and close, you can calculate your hull speed by 1.34 times the square root of the waterline length of your boat, for displacement, non-planing hulls.