why does the 0 tx block have to come "immediately" after a large block?
They don't. Empty blocks can come after any sized block. But I just showed that F2Pool is
more likely to produce an empty block / "defensive block" when the previous block was large than they are when the previous block was small or medium.
it might be interesting to see if Antminer becomes statistically significant after 2 blocks instead of 1.