Although not evidence either way - were apple to include mitigation for row hammer its most likely they would advertise it as they do their other security infrastructure. Its about on-par with the encryption processor in the imac pro in terms of technicality and that is public information.
They did increase their DRAM refresh rate and mentioned it in release notes. There is not much more you can do easily - and it comes at a real cost in power usage.
You can actually refresh specific rows adjacent to frequently accessed ones by issuing ACT->Precharge command combos. Timed right it only comes into play during attacks (whether intentional or by coincidence) and therefore has little additional power usage.
Also that release note was for the Mac not the iPhone. They are less likely to apply that mitigation to the iPhone precisely because it would result in increased power consumption. They could however integrate the more advanced fix in the memory controller since they control the SoC. I would expect them to brag about it if they had.
