The paper you cite is from 2014 and the mitigations discussed there have all been circumvented. [1] is from 2020 and a better read for Rowhammer mitigation.

[1] J. S. Kim et al, Revisiting RowHammer: An Experimental Analysis of Modern DRAM Devices and Mitigation Techniques https://arxiv.org/abs/2005.13121

Thanks for pointing that out!