Atomic radius of Silicon atom is 0.11nm, which results in width of about 0.22nm. So even tightly packing atoms will make the barrier only ~13 atoms thick.
Van der Waals radius is about twice that, resulting in 7-atom-wide barrier. Quantum tunneling [0] is apparent at 3nm, and gets worse from there, so I don't see how they would be preventing electrons leaking through the barrier, unless "3nm Fab" would have a shovel of marketing salt to boot.