Closing a long/short position by selling/buying bitcoins has been possible since the beginning:
it's "hidden" in the "my margin positions" page, if you have "expert mode" activated;
user experience might not be at its best, I know, but the feature is there and will be
improved over time.

If by "kraken already implemented this feature" you mean "commission free closing of positions",
then it's not: in our case, you still pay for the trade. But we currently do not ask any fee to
maintain the position opened, so grab that while you can :-)

/paci