Let's just say I wouldn't use the spread of allocations across address space as a PRNG. Depends on your allocator of course, but you should normally see some fairly linear patterns. The allocator can be very simple if all (or almost all) allocations are the same size, so you can easily optimise for cache efficiency.

I suspect the cache angle they're playing up refers to memory footprint of each cell. I don't know how big their cells are, they could be anything from 2 machine words up. Integers are supposedly directly represented, and if they've embedded their GC liveness info into spare bits (or stored it elsewhere entirely) then that makes it extremely compact and thus cache friendly.