I have heard the myth before, but never from anyone with any experience in TKD, and certainly never from a Korean.It was a question. It was just something I had heard from a friend who did Tae Kwon Do. I was just trying to verify the validity of it.
I have a copy of Muye Dobo Tongji, and there's certainly nothing in there about dismounting a rider with a kick.
Recently attended an evening of traditional Korean games, and it was surprising how many of them are based around use of the feet. Use of the foot for purposes other than walking does seem to be part of the culture outside of Taekwondo.
Take Jegi Chagi for example.