My car has that extra stalk but it doesn't seem to slow down or 'adapt' to other cars infront of me
Mine too.
My curiosity was piqued over this so I checked the user manual (when all else fails, read the instructions
). Its a little vague but page 40 suggests that passive cruise control can either come with a third stalk (as in your picture) or controls on the indicator stalk.
ACC (pages 187-194) says that this system always comes with a third stalk. As
@'Little' John says, the difference is that there is an additional distance control switch on top of the stalk with ACC.
Checked my car and it definitely has no radar so its not active (as was starting to think there was something I hadn't discovered for a minute).