The fourth paragraph clearly states that the original Zerocoin approach has the flaw you thinking of. However in the last sentence it says that Anoncoin will use RSA_UFOs and generate them by using Sanders 1999.

Can you even read?

EDIT:
Of course you have to trust Gnosis that the coding is correct. But you need to trust any other developers as well when you use any program on this world. Like I said before, If you don't trust anybody you will live in a box.

EDIT2: Other people don't need that trust, they read the opensource code