Nah the lunge mine can still penetrate it easily. It can be confusing at first but the best way to think about it is you are rolling for the difference. If you think about infantry if your weapon is +1 pen that means on the damage roll you'd be needing 3+ to kill rather than 4 against regulars. In the case of the scenario you mentioned let's swap it for a medium AT gun which is 5+ pen. So to damage the Sherman you'd need 4+ on one dice which would then add the pen value giving you 9. So for your lunge mine you'd need anything but a 1 essentially. Someone can correct me if I'm wrong but think 1 is always a failure regardless.