The C-17s are designed for STOL operations and can fly in and out of dirt airstrips.
The article states that the airport it landed on was Peter O Knight Airport, which has two runways, 3580ft and 2687ft long according to this site:
http://www.airnav.com/airport/KTPF/
It doesn't state which runway it landed on, but if it's the shorter one, then that is quite an impressive feat!