This is CryEngine 3 with some additions, right?
“Large nominal units” is very different from “large scales of objects” – this demo shows mostly the former, which really isn’t that hard with some camera/perspective tricks; the latter is much harder. Seamless distance travel is largely a question of defining transition points and shifting/scaling coordinate systems.
To be fair, Kerbal Space Program (version 1, the only good version) also does most of this 
– on top of Unity! It’s more of a production problem, than an engine problem. (But the 64-bit coordinates we got in 5.0 help!)
Having built a full-earth simulation and rendering MMO engine in the past (sold to the US Government) I can tell you that normal players don’t actually want “earth scale.” The earth is massively, vastly, full of stuff that has no gameplay value. And there’s no reason a “virtual” earth should be the size, or even shape, of the real earth – the whole point of “virtual” is to make something that works better for some use cases than “real.”
Real distances between cities is boring. Real city sizes are boring. Real planet sizes are boring. Luckily, this game has compressed most of that, as will any game. They also have a lot of content at this point, and content is king! I think the Star Citizen developers are cool for chasing their dream, despite all the various ups and downs, and I’m happy to see them finding ways to keep that going, but if I were to build a universe-scale game today (or even just Kerbal Space Program 3) I’d do it on Unreal.