I think that Sarek's point was that Spock's effort to control his anger toward Nero had been the cause of his lashing out violently at Kirk. Trying to internalize an uncontrollable emotion resulted in redirecting that emotion in an unhealthy way. I think what Sarek says essentially means that it's OK to direct his anger toward Nero.
This is a more subtle treatment of Vulcan emotion. Vulcans don't simply suppress emotion at all cost, but rather as Sarek said to young Spock they practice "the control of feelings, so that they do not control you." Sometimes even Vulcans have to realize they can't simply entirely suppress an emotion, they have to acknowledge it and deal with it.