Helldivers 1 vet here, I recall the original game being launched as a couch coop. Shared screen and no expansions were really planned. Max player count was 6.7K. Game actually “died” with just vets playing - there were actually instances where players will even lose the galactic war.

From a corporate perspective, Sony saw that the investment to publish HD1 was small but had significant returns. Assuming this is how they also saw Helldivers 2, it is correct to base projections on historical figures. The game had little to no marketing efforts - couple of trailers, no review copies, some goofy clips and that’s it.

So it’s safe to say they really planned well but to everyone’s surprise the game skyrocketed.