How so? How can making an online mod can be considered as "supporting piracy"?
Based on this logic, we all paid AAA price only for MP then - since it becomes piracy supporting only if it's MP mod. Literally, all other mods are playable with the pirated version.
This is just another mod - nothing more. And code-wise, it's independent from Bannerlord. I.e. if you make a game, let's say Bannerlord clone with exact same endpoints files etc, you can run this mod without needing any BL at all. If TW had some custom server modules and this guy tweaked it in a way that pirated games can also play, then it would be an issue (which, iirc was the case for BF:BC2 mods) But this is not the case.
So, as far as I can see, TW cannot do anything to this mod in legal ways - but they can prevent for this mod to work. Which would be a desperate effort but wouldn't surprise me if TW does that.
Also, shutting this mod via lawsuit won't end privacy for Bannerlord. Because it's not related and I think TW knows that. They can use this as an excuse to justify their extra effort to "block" a mod though, as I mentioned above.