I am not a developer of the software, so please wait for in-depth reply from someone with developer team or knowledge.
However, as an user point a view, I can confirm, once installed, NextCloud doesn’t need any active Internet connection to operate within LAN
Home page displays error on weather update and app store won’t function due to unavailability of an active internet connection but apart from that, installation and other functionalities works just fine.
If you may notice, there is an app store for 3rd party app, and few information like weather update and such (in default installation) requires Internet to fetch their respective data. Apart from that, NextCloud is very much usable and works fine within a closed network.