I have not heard of this, but It might be true?
The reason you have the vent pipe over the tank is to stop water going in it, the header should be very low in water when cold, because it warms up and expands, so rises up the tank, so only have the tank half full when cold at most.
If the vent pipe was to be fitted to the tank, then if your Ballcock failed, and say your over flow was blocked then you would syphon cold water down the vent on to Hot, this would not be good.
Just had a quick Google, on the 2001/2002/2005 changes in regs and it does not mention this from what I can see.
Hope it helps.