They're the same thing, and require the same level of attention to use safely. The only difference is that Tesla didn't enforce that level of attention because they're more willing to feed their drivers to the undersides of trucks at 70 mph and then blame the driver for not paying attention than Mercedes or Volvo are, and the press and HN are more willing to let them get away with it.

Do Teslas now drive around on their own? I seem to have missed something.

You haven't missed anything, they use the same system that's available in Mercedes and other cars. Some people just misread the announcement and we've been living with the misconception that Tesla has self-driving cars ever since.

I don't know why, either, since this would presumably be very easily debunked.