User blog comment:Lupercalus/Savitar/@comment-27700324-20161204012101/@comment-27700324-20161205041151

He knows because he can go into the speed force. If you can do that, you are probably able to see someone travelling through time and changing something in a big way. It is also probably similar to the comics cause this Savitar has a cult too. Only difference is Dr. Alchemy being part of it.