It's non-obvious because it's a bad idea. This paper comes from a wacky world where latency and power consumption don't matter. The comparisons between CPU vs. GPU aren't that compelling just on the surface of it. The latency/power consumption numbers (compared to dedicated ASICs for this sort of thing) are just laughable.

Being the most compelling 'software router' is sort of like being the 'tallest midget' but even in this domain, I think their alleged advantages over CPU-only are mainly due to carefully massaging the presentation of the data.