From the original paper, "For example, SECDED (single error-correction, double error detection) can correct only a single-bit error within a 64-bit word. If a word contains two victims, however, SECDED cannot correct the resulting double-bit error. And for three or more victims, SECDED cannot even detect the multi-bit error,leading to silent data corruption. Therefore, we conclude that SECDED is not failsafe against disturbance errors."

The challenge for the attacker is to produce a 3 or more bit failure before a 2-bit failure occurs, which would trigger some alarms. I haven't seen any research on the feasibility of such an attack.

Additionally, there are stronger ECC schemes than SECDED, for example the one in IBM's ChipKill. It is hard to imagine a RowHammer attack that could remain undetected with such a scheme.

It is fair to say that ECC memory raises the bar for attackers.