The distance between Okinawa and the Miyako Islands is just 150 miles, close enough by FAR to guarantee a costly air/sea battle with Japan.
Were China to take the Miyakos (totally uncontested Japanese territory and home to maybe 40,000 Japanese and featuring a 7,000 foot airport, quite easy for an Il-76, etc) they would almost certainly have to take Okinawa at the same time, in order to thwart an easily mounted counter-attack from there.
Of course, that would involve the USA.
Thanks. I wasn’t quite sure of the reasoning.