Eh, I think that's underplaying it a bit. Of course you can't literally turn a state the size of Texas into a plate of glass with nukes, but a couple thousand dropped in the most economically productive parts of the world?
In terms of the nuclear blast and radiation itself, it's true that probably only a half a billion people or so would die. But you'd be essentially ripping out the entire economic and political nervous system of the world, resulting in massive economic dislocation and political chaos. I find it plenty plausible that that by itself could easily get the body count up to the billions.
There are also possible environmental effects: the dreaded nuclear winter. It's not clear to me if the debate was resolved one way or another about how bad it'd actually be, but it seems like a real possibility that it'd hurt the climate much worse than even the most dire warnings of global warming. And it'd happen more or less instantly, as black soot would immediately be sent up to the stratosphere.
An article from 2007 in the Journal of Geophysical Research, courtesy of Wikipedia, states:
A global average surface cooling of –7 °C to –8 °C persists for years, and after a decade the cooling is still –4 °C (Fig. 2). Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about –5 °C, this would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature changes are largest over land ... Cooling of more than –20 °C occurs over large areas of North America and of more than –30 °C over much of Eurasia, including all agricultural regions.
That's bad news, and would compound the political strife, wars, and economic dislocations. And while now refugees from war have to possibility of fleeing to a neighboring stable country, with this scenario pretty much everyone is in a terrible place, so you don't even have that possible pressure valve.