hence why I used "" on the word past and also why in this issue Eggman pauses with an "Er" before saying "Younger".
It's more that Eggman and Robotnik Prime share the same history up to a point, somewhere just before or during Endgame. There they branch off, with Prime having died to the UA, and Eggman having lived. Eggman then roboticized himself and waited in some satellites. A few years later, he killed his Sonic and then went back in time or something when travelling to the Prime Zone.
Basically: Naugus knew a different Robotnik, but the two Robotniks experienced the same events.
Well, the real reason Zonic isn't going to take Eggman back to his home zone is because he already filled in the gap that Robotnik left behind. Also, like I said before, Robotnik isn't quite dead; just stuck in a void between time and space. This also prevent the Zone Cops from going into there and bring back Robotnik; they have only have the technology to go through universes, not dimensions.
When Robotnik came back his molecules were unstable anyway.