I'm more surprised that the button is defined using a <div> tag instead of a <button> one. http://semantic-ui.com/elements/button.html A lot of these elements seem to be using div tags instead of list, button, ...
“Semantic is tag agnostic, meaning you can use any html tags with UI elements.”
It's unfortunate that they chose to use un-semantic tags in all their examples. It's almost backwards. Other than that, it's extremely well documented and a pleasure to work with, and much easier to pick up than Bootstrap these days.
So is it the obvious usability fail that this statement would seem?