I think that I got what they did:

The 2x refunds correctly done was only for THEIR OWN WALLETS.

I mean, a Ponzi scheme will give to the old ones the money of the new ones, right?
So, if "the old ones" are mine, I will get more money by using credits of "the new ones".

When coin2x filled their accounts, mission accomplished.
And because all the transactions are public some people thought (me included) that they were actually giving 2x profit legitimately.

So, that "they have only 12btc balance!" is erroneous, they were stealing all the time!, all the double profit was only for them in the beginning. All of those confirmed transaction were only for their own people!

It is a remix of a ponzi scheme, adding own zombies to the start of the chain in order to obtain lot of fast money.