ECC fails to prevent rowhammer in more or less exactly the same way that address space randomization and similar hardening techniques fail to prevent stack smash attacks.
Which is to say, it basically prevents rowhammer in a practical sense. You can flip the bits, but you don't have the level of control needed to flip the right bits.
I'm not sure this actually makes them wrong, if you take their statement as "ECC makes the attack more difficult/ statistically less likely". The attack you're describing is sort of akin to a heap spray attack to improve your chances of hitting your shell code.
But dropping the analogies entirely and stating explicitly is a lot better. ECC makes rowhammer less likely to succeed and slower to exploit.
Which is to say, it basically prevents rowhammer in a practical sense. You can flip the bits, but you don't have the level of control needed to flip the right bits.