Board Thread:Questions and Answers/@comment-39303455-20190430174357/@comment-39829766-20190616150104

This image makes the most sense to me. (please view it, please)

Essentially the "he can perform actions that he would've been able to perform anyway" that others have mentioned above. Epitaph's predictions are absolute, with the exception of Diavolo, who can escape his own fate. If his future includes him throwing a rock, that rock will be thrown - but Diavolo can erase time, so he can just walk away and never touch the rock.