My guess is that the typical group is GM + 3 or 4 players, meaning they almost guarantee a sale of the Adventurer's Toolkit with every box of the main game they sell at the additional printing cost of a few Progress Tracker pieces and an Activation Token.

At least I can't find any reason the game is "optimized" for 3, so far... there's no encounter balancing system like in DnD for example. The only reason it can't be played with more than 3 players out of the box seems to be a lack of tokens.
I think you'll find, as you read, absorb and conceptualize the game, that it's considerable less boardgamey than it looks. Particularly after reading the Tome of Adventure and the sections on campaigns and scenario building - it's very "story" focused, and the mechanics seem much more focused on providing seeds for narration and story than providing a framework for highly tactical play.