While it seems odd, it makes more sense once you realize that TaleWorlds is not actually developing it. The company supports its mod community and, for the second time, decided to promote one into a full DLC. The previous mod was Napoleonic Wars, developed by Flying Squirrel Entertainment, which is now a full independent game studio.
The expansion, Mount & Blade: Viking Conquest, is the commercialized and updated Brytenwalda mod. Being an external effort, I doubt that TaleWorlds diverted much resources away from Mount and Blade II: Bannerlords to release this expansion. As an added benefit, it might launch a new independent games company — maybe even a virtual furniture and meatball franchise.
While the company has not announced online player counts yet, this engine is known for supporting hundreds of players. Napoleonic Wars regularly has servers with 200-player caps not including horses (although I have heard, but not seen, that people have pushed that up to 250). This could be very interesting for a Viking Age theme.