But in practice "centrist" news sources are far more likely to present multiple points of view -- e.g. a left and right one -- while "left" and "right" news sources generally do not.
But at a deeper level, centrism isn't really an ideology at all, in the way the left and right can be. You can be a hard-core leftist or you can be a hard-core conservative, but the idea of a "hard-core centrist" doesn't really exist.
So I'd argue centrism can be the opposite of bias in a very real way. It's about dropping bias towards ideologies, and treating issues in a practical balanced way.
No, in practice less strongly biased sources are more likely to present multiple points of view. Now, the same amount of variation can seem more diverse when you tend to bucket things into “left/right” binary categories, if the center of variation is near the point where you draw the line between buckets.
But that's an artifact of forcing things into binary buckets making a centrist outlet providing center-left to center-right views look more diverse than a right-wing publication providing center-right to far-right views.
No, centrism isn’t the opposite of bias.
Centrism is a position toward which there can bias of any strength. Position of bias on a left/right (or any other ideological) axis and strength of bias or two orthogonal, continuous dimensions.