The difference is that this AI agent was built to beat humans. Horses were not built to run faster than humans.
You have to remember the specific comparison and the art that humans tried to partake in to master go.
Humans do not run to beat horses. We never compete. That would be silly.
Cars and humans also do not compete at the 100 meter dash.
If humans and robots compete at 100 meter dash than the olympics will similarly lose its luster.