Yes, it is using the right affordance (to be consistent with OSX Lion).
This is slightly confusing implementation of what I think they are trying to achieve. I agree, it does look like the slider/switch control used in iOS. Sad news is, this is what OSX Lion is going to look like. They are changing the look of these "tabbed" controls. See below:
At this point, I agree it's a little confusing, considering that iOS uses a similar styling for their slider controls. Only time will tell if this was the right move on Apple's part.
As far as apple.stackexchange goes, they are doing the right thing by reflecting the upcoming Apple interface in their own design.
Update: As Jin pointed out, Apple reversed this design decision in the latest build of OSX Lion (thankfully). You can read about it here. It appears that apple.stackexchange will also be changing it to reflect this decision.
The importance of user feedback in action! :)