Certainly the way it's been reported gives no way to know whether it was a good or bad piece of research.
What teenagers remember about what happened to them before the age of 6 may be patchy - some might accurately remember how often they were smacked, while others might have been smacked often before about 3 and not remember it, or have been smacked once at age 5 and remember it vividly, assuming it had happened often because of the strength of that one memory.
There are no other listed measures of what the family was like - as Cory says, a family which smacked (in the US 10 years ago) might have been differed (on average) from a non-smacking family in many ways. You might figure they were more authoritarian in other ways, you might figure they were less caring, or more violent, or all kinds of things - we have no way of deciding that from the reported result.
And then there's things like "those smacked from 6 to 11 were more likely to get into fights" - well, you could equally say that children of 6 to 11 who regularly got into fights tended to be punished differently from those who never got into a fight.
Useless as reported. Possibly useless in the actual research too, but no way to tell...