The article says: The act of undoing need only become part of our linear history if we squash the proverbial butterfly and thus alter "the past"; in that case

The article says: The act of undoing need only become part of our linear history if we squash the proverbial butterfly and thus alter "the past"; in that case the act of undoing itself is instantly recorded as a series of changes, by replaying the redo stack onto the undo stack twice: 1) Forwards, for the original changes 2) and then backwards, to record the history of our undoing. By squash the proverbial butterfly and thus alter "the past" they mean make an edit after undoing.