Earlier quoted context omitted.
If historical claims are valid, then Mongols would be very happy to claim large swaths of land. Or if more recent claims are to be taken, then the Brits have claims over quite a large amount of countries. Historical claims are meaningless and are just an excuse for expansion.
I did not comment on the "validity" of the claim, just explained its rationale and history. Chinese territorial claims in general are not "an excuse for expansion", they are rooted in territorial losses at the end of the 19th century and during the revolution of 1912 with the formal aim of recovering them. They also predate the PRC as you'll find that the ROC/Taiwan has the same claims for the same reason. This does…
Then why make a claim? Claims are made to prepare the domestic audience so that when war comes there is home support for the action. It is not made lightly.
The Chinese are definitely taking action in the South China Sea. It is not just words.