Is this mod compatible with Open To LAN and is that officially supported?

You mean vanilla option to open your singleplayer world to LAN, or what?

...why would it be incompatible. Lan servers are not very different from dedicated servers in this regard.

yes the built-in open to lan button.

it didn't seem officially supported. has it been tested for that? it's also not written anywhere on the mod.

There's absolutely no reason for it to not work. Considerations you have to do for LAN server are the same as for dedicated one. I might document it later.

you can say "I didn't test that use-case", it's not gonna harm you...

If you really wanna hear that - yes, I didn't test the use-case. I still can confidently say there is nothing different about such use-case that would make it not work as it does in use-case of dedicated server.