USB 2.0 has max rate of 480 Mbit/s but normally goes maybe around 25 MB/s or 192 Mbit/s...
I usually get around 30MB/s out of most of my external USB2 drives. Depends on the HDD, SATA-USB bridge IC/firmware and host controller.
Not sure why you are confused about USB2's bandwidth and the router's wireless claims. While USB storage may be limited to 480Mbps on a good day or 240Mbps typical, the router is also connected to 1Gbps WAN, 5x1Gbps LAN, 866-1300Mbps 5G WLAN and 450Mbps 2.4G WLAN so there are plenty of data sources/sinks that can be combined to hypothetically fill up the 866-1300Mbps WLAN aside from USB2.