The letter references a number of items besides the printable gun, including "125mm BK-14M high-explosive anti-tank warhead" Which sounds rather closer to matching the intent of the munitions export regulations.
You can pick up most of what you need to know about anti-tank warheads from a Tom Clancy book, or any library book about shaped charges.
For most people an anti-tank warhead is going to be useless in the context of how to disable a tank.
http://en.wikipedia.org/wiki/Explosively_formed_penetrator
Of course, perusing the above link, it would seem the main limitation on proliferation is chemical synthesis of the HE, not 3D printing technology.
As far as I know it's difficult to 3D print 88mm cannons.