The "unlimited" aspect they refer to is that their engine can render geometry at whatever level-of-detail without significant performance differences. Whereas with polygon based renderers the polycount has a direct influence on performance regardless of the area it occupies on the screen.
So let's assume it's just 1 mm of ground and no other objects exist. That's still 64.000.000.000.000 atoms.
I doubt they store their data at a constant resolution ( at least in compressed form ). Regions of molecules that have the same content ( air, soil etc. ) don't have to be stored individually. A 2D representation ..

Can I trust that it's now running on software only and not GPU considering I know they're dishonest about other stuff?
Current GPUs aren't of much use for what they are doing actually.