Over the 162-game regular season, there's time for the elite teams and players to shine; in a five- or seven-game playoff series, however, any team can beat any other team. That's not chemistry; that's good luck.
Perhaps the greatest blow to the chemistry argument, however, comes when one considers the accomplishments of the above players.
Henderson has won a pair of championships (and has been to the playoffs on other occasions, including maybe this year), while Belle and Bonds have each made multiple postseason appearances.
And I'm not even going to mention Dennis Rodman.
Mattingly? The closest he came was last year's loss in the wild card series against Seattle, which the Yankees really reached in spite of him.
I don't mean to pick unduly on Mattingly, and it's certainly true that winning and losing are team efforts.
But it's clear that the road to success in sports is paved with talent, not sound bites.