The more I think about it, the more I feel like something's not right about the future Jon described.
Think about it. He mentions that they've been allies with the Subterranosauri for 20 years. We can assume Jon is like... 16 or something, which would mean he would get born 4 years from now.
It also means they should have been fighting Hank for 20 years because Hank is destroying everything now. But that wouldn't make sense.
Not when we know that in this future both Clark and Lois die when Jon is little. But based on the photo we get to see, that doesn't look like a stressed out Clark and Lois who have been fighting Hank for years now.
The timeline just doesn't add up. It would make much more sense if Hank being attacking Metropolis after Jon was born, not 4 years before.
So, I think that Jon didn't travel back in time- or really just travel back in time- but to an alternate reality. One where things are different and Hank went rogue much earlier than in Jon's timeline, or wasn't this cyborg Superman until like after Jon got born.
Could I be wrong? Duh, of course. But that doesn't change that something's wrong.