Let me clarify what it means to merge mine a coin with itself, since this is at the centre of the attack.

A merge mined block consists of a parent chain, and one or more auxiliary chains, both of which are covered by the same proof-of-work.  Normally, both/all chains correspond to different coins.  For example, you might have a bitcoin parent chain with a namecoin auxiliary chain and a ixcoin auxiliary chain.  But there's nothing to prevent you from having multiple chains belonging to the same coin.  So you could feasibly create a block with a namecoin parent chain and a namecoin auxiliary chain, or a block with a bitcoin parent chain and two namecoin auxiliary chains.  By merge mining this way, you can mine two chains at once from the same coin.  This allows an attacker to build a chain in secret, while still generating revenue mining on the public chain.