He did not, all the AMD overclocking was done via CCC which doesn't include over-volting, Lius increased the clock speeds & the power limit, the power limit is in no way associated with over-volting, it only prevents throttling once the GPU exceeds its designed TDP.
The only over-volted card is the 660 Ti, Linus uses the GPU Tweak tool from EVGA to overclock all the Nvidia cards GPU Tweak does have a voltage control slider, however it's either locked or extremely limited on the 600 & 700 series cards, only the 660 Ti from MSI had a generous overhead because it's the power edition which MSI unlocked the voltages on.