From my experience, that is not exactly it. It's not confusion of social signals per say, rather it's the amplification of these signals that causes anxiety. It's the over analyzing and excessive processing the brain does that causes anxiety.

From my experience, MDMA eases the mind, and fills it with empathy, love and connectedness to others (or release the molecules necessary for this). Under this state, it's not hard to see how one could temporarily break free of their 'normal anxious' state.