Those docs refer to ephemeral storage. I haven't seen any claims by Amazon or test results that indicate it is also true for EBS volumes. I have been meaning to test this myself.
gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 344.745 seconds, 31.1 MB/s
real 5m44.748s
user 0m0.025s
sys 0m10.152s
gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 217.106 seconds, 49.5 MB/s
real 3m37.125s
user 0m0.023s
sys 0m10.268s
gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes (11 GB) copied, 247.319 seconds, 43.4 MB/s
real 4m7.323s
user 0m0.036s
sys 0m9.794s