I have helped ship several aerobatic aircraft in containers including my own one. I have seen between 5 and 8G on the G meter when the containers were opened. I think others have seen more.
I would therefore think that this was a bit optimistic:
was rail could cause 4G forward/backward, and 1/2G sideways. Road could cause 1G forward/backward, and 1/2G sideways. And sea could cause a maximum of 0.8G in any direction.
I think the loading and unloading (dropping) probably give the highest load. We ship with the aircraft suspended from the wing mountings as most undercarriages aren't good for 6 G.