"By not filling first baseman Chris Davis’ roster spot when he was placed on the 15-day disabled list Sunday, the Orioles had the option to re-sign Pearce and add him to the active roster after he became a free agent, but before they went back to a full complement of players."
I think I can reconstruct the timeline from this sentence. They released Pearce but when Chris Davis was injured, they put Davis on the DL without replacing him and took advantage of the window that allows you to bring back a player quickly if it's a DL replacement. In this case, that also involved re-signing him.