The point wasn't that certain people's riding is art, more that it's like art; it's about expression and creativity and challenges the way we perceive our surroundings and our sport. The following definition of art as "the expression or application of human creative skill and imagination, typically in visual form" kind of goes against the point you're making as well, but I do see the point that you're making I do, in part, agree.
I don't necessarily agree that if the creator of something doesn't consider it art then it isn't, though. What about the audience, beholder, viewer, whatever, and their view or interpretation of what they are seeing? If they consider it to be art, whether or not the rider, painter, sculptor does, surely then it is art to them?