Yeah, so the "Seed" can me different for everybody.
AND when you send yours planet in the atlas, it get load randomly into other people games.
The 18 quintillion is just a number of possibility, not the universe totally mapped.
Just like when a game come out with heavy customization. And they say : 2million possibility.
I just "hate" to see people over-hype the game, as "There is a entire universe in this game". It's not lol. It's just random generated planet from seed. They "pop-out" as you get to them.
And that why the game is not really MP. That you "can't meet" because there is not a "global universe" where all the seed/coding planet are the same at the same spot.
I guess if there is some MP it will be at the center of universe, or some "game-event" MP oriented.