Airports not show in ground network

What is the policy when listing airports in ground network? For example, KNH airport listed following airports in its ground network:
CYI Taiwan 232km
LCX China 219km
LZN Taiwan 246km
SQJ China 244km
RMQ Taiwan 221km
TNN Taiwan 270km

But why not listing more closer airports like:
XMN China 16km

In real world, there is ferry connecting KNH and XMN. (