Actually, Xi did not back down, as noted by the Guardian, in July 2016, re the ruling by the International Court in The Hague against China on these disputed islands:
'The Chinese president, Xi Jinping, said China’s “territorial sovereignty and marine rights” in the seas would not be affected by the ruling, which declared large areas of the sea to be neutral international waters or the exclusive economic zones of other countries."
This was another one of Obama's failed red lines.  Sending a few destroyers through has done nothing.