"Very difficult" is context dependent. It might be difficult for you and me to backdate a tweet, but easy for a Twitter employee or a hacker with a 0-day exploi

"Very difficult" is context dependent. It might be difficult for you and me to backdate a tweet, but easy for a Twitter employee or a hacker with a 0-day exploit. You also introduce the risk of your message getting deleted, for whatever reason. And these ways may also break your anonymity or leak your exploit if you're not careful. A blockchain backed timestamp is simply better.