NO OFFSIDE IF ALL OPPONENTS ARE IN OTHER HALF?

Question:
The prof in a referees’ clinic explained that if all the fullbacks on a team move into the offensive half of the field, then the opponents are freed of any offsides restrictions at all. I’m dubious; I don’t see how this follows from Law XI. Can you explain it to me please, or perhaps the prof is mistaken? Thanks!

USSF answer (August 8, 2010):
You are pulling our legs, right? It makes absolutely no difference where the players on the defending team are, the player in the opposing half of the field must still be no nearer to the opposing goal line than the ball to avoid being in an offside position. On condition that we specify that the attacking player is on the halfway line, he can never be either offside or in an offside position when the ball is played by his teammate in this particular scenario. The Law is clear (emphasis added to ensure understanding):

LAW 11 – OFFSIDE
Offside Position
It is not an offense in itself to be in an offside position.
A player is in an offside position if:
• he is nearer to his opponents’ goal line than BOTH the ball and the second-last opponent
• he is nearer to his opponents’ goal line than both the ball and the second-last opponent
A player is not in an offside position if:
• he is in his own half of the field of play or
• he is level with the second-last opponent or
• he is level with the last two opponents