I dunno, 90% of the time someone makes a statement like that, they're wrong. Without being privy to the codebase, it's impossible to know!
Moving ships are different from planets; planets probably don't move. Gravity isn't the issue. The more likely problem is when you have two very close objects, moving at high, but similar speeds, in a discretized simulation, you get very large problems from very small inaccuracies, and the whole thing ends up unstable. This problem is much worse when you add in network latency! Anyone who's played space engineers will know what I'm talking about here.
Fortunately, the solution exists; use relative frames of reference (ie, players inside a ship have their movement tracked relative to the ship, as a few posters have already alluded to). It's probably a bit tricky to implement, but given what we've seen so far, I'd be shocked if Novaquark didn't have the technical chops to do it. Nothing here to worry about, I reckon.