1. What does "offensively" mean? The translation is "in the position of in an offense position in baseball", but I think if it's "aggressively" as in a dictionary definition.
2. Why isn't it simply "held" instead of "held for"?
ex)Stanley Frank Musial was born in Pennsylvania...in 1949, Musial finished one home run short of winning baseball's Triple Crown. After struggling offensively in 1959, Musial utilized a personal trainer to increase his productivity until deciding to retire in 1963. At the time of his retirement, Musial held for seventeen Major League records and nine All-Star Game records...
There is a defensive and an offensive (called the offense) side in baseball. According to this text, Musical had problems while on the offensive team. The meaning of the offense in sports is related to being aggressive in that the offense has to be bold and initiate moves to win the game. The second part of your question (held for) seems to be a mistake.