This is an expression from baseball. It means he hit the ball so hard that it was out of play, thus automatically scoring. Actually,it's the best move one could make in baseball. So I imagine that they mean his estimate was exactly right.

--------------------------------------------------
Note added at 1 hr 35 mins (2005-12-25 11:35:22 GMT)
--------------------------------------------------

Or I suppose it could mean that his prognosis was way too high / exaggerated. Depending on context.

--------------------------------------------------
Note added at 1 hr 48 mins (2005-12-25 11:48:33 GMT)
--------------------------------------------------

Yes, I think exaggerated is more likely. See this explanation for *ballpark figure":
Definition:
A good numerical guess; an estimate.

Example:
1) I'd say two hundred dollars, but that's a ballpark figure.

Etymology:
This phrase is related to another popular phrase, 'in the ballpark', which means 'close but not yet there'. The idea is that you can be in the ballpark (a place where baseball games are played) but not yet in the right seat (the seat you have a ticket for). The seat is a metaphor for the right idea or place, and the ballpark is a metaphor for an approximation of that idea or place. A 'ballpark figure' is a number that is 'in the ballpark' (close to the true number) but not quite in the right seat (the true number).

http://www.englishdaily626.com/slang.php?014

So forget the Volltreffer. Übertrieben ist eher wahrscheinlich.