To "nerf" means to weaken or make less dangerous. Google's forced obsolescence of Nexus devices is clearly making the devices weaker and less capable than they can be. I think the use of the word "nerf" was accurate and justified in this case.
Still, I don't think it's super appropriate, because there's nothing making the older phones worse, they just aren't getting better in the ways they obviously could.
They released an update to the Galaxy Nexus that literally halved performance across the board and never addressed it. The Nexus 4 has had similar woes.
I don't think it's necessarily planned obsolecence so much as allocating less resources to testing and fixing issues on older devices, but to an end-user the result is similar.
Galaxy Nexus was going to be supported by Android 4.2 (I was really annoyed that it wasn't!) but the fact that it's not has to do with TI refusing to update their closed-source drivers for their OMAP 4 chip to be compatible with the Linux kernel used in 4.2. By the time people figured out how to update them, Google had given up on providing support. I'm still pissed about that.