the stand is explained pretty well in the anime imo
the meme about him being confusing just originated from poor translations lol
Epitaph allows Diavolo to see 10 seconds into the future.
King Crimson erases 10 seconds of time
With Epitaph, Diavolo can plan his actions accordingly based on its visions within that 10 seconds.
Since King Crimson's erased times prevents people from remembering their actions within that time, they cannot act upon or change their action that was foreseen by Epitaph.