I was under the impression that rowhammer could work because ECC ram can't correct for a high number of errors. Specifically "However, even such modules cannot correct multi-bit disturbance errors" from http://users.ece.cmu.edu/~yoonguk/papers/kim-isca14.pdf.
With such a crude mechanism the odds are pretty slim that you can create all the right multi-bit flips in one go without hitting an intermediate state that triggers an ECC fault. Research theorizing and actual practice aren't always the same.