Notice that the breadcrumbs section (which lists the parent topic names all the way to the root topic) has a drop down that pops up (that's a funny expression) for each parent topic which lists all the peer topics at that level. This is a cool information architecture.
the usability of the implementation is bad. Notice that the pop-ups cover other topic names and its nuisance to have to move the mouse or hit escape button to lose the pop up that popped up because you overshot the mouse.
May be the usability testers tested with a few topics in the crumbs. The problem starts when there are many topics. Whats the way out? Nothing easy. Thats probably the reason why it is deployed that way.