These missiles, which may be only on Woody temporarily, only augment several J-11 interceptor aircraft that have been based at Woody's airport for months and Chinese warships (armed with SAMs) which have plied the Paracels for years.
B. Woody Island South China Sea. China calls it Yongxing Island. (This satellite image courtesy Digital Globe via defencetalk(dot)net)
Woody Island towards top. Chinese miltary aircraft from China's Hainan Island can land at the growing air base on Woody Island.
Woody Island already sits in a highly congested strategic area bristling with weapons. So a staged show of missiles on a beach is only an additional threat, not a game changer.