I route gigabit with a much slower processor than that.

Good routers are built for reliability, and most of them seem to opt for slowish CPUs and a fast-path for routing that doesn't involve the CPU at all. The CPU is there to update the routing table and do other supporting chores.

You can find much slower machines able to route gigabit, the limitations are mostly on the bus and ability for the driver/chip to efficiently process notifications. We're only talking about 125MB/sec, and if any of that is bulk data utilizing the full MTU, as few as 81k packets/sec, represented in (modulo driver/chip/config) far fewer than 81k interrupts/sec

You have to look at the architecture. They are using a dedicated switching chip that has multiple gigabit lanes. While routing is a different path in software most home and SOHO implementations have very few routes to deal with. That being said the switching can offload the majority of those lookups. It's far more than powerful enough to handle what it states given its architecture.

Their website says "1.6 GHz dual-core ARM", which sounds decidedly more powerful than an iPhone 3G