This thread takes me back! I recall a very similar discussion in pages of the British Hang Gliding Association magazine in the 1980s. Being pre-internet, it raged on for months in the correspondence pages, and someone who ought to have known better even wrote a stroppy article rather embarrassingly advertising his ignorance. It is sad to hear that decades later:
it is still a common misunderstanding with hangglider flyers
Of course, hang gliding is particularly prone to speed perception errors when face down and turning low over a hill in a strong wind, which has unfortunately led to more than a few accidents, the cause of which wrongly attributed by those a with a dodgy grasp of basic physics.