Valve keeping up with the trend of "worst kept secrets"
Well it's very intentional I believe. You can invite an unlimited number of friends and there's no NDA or anything, they just casually ask you to not share details about the game since it's still a work in progress and in alpha, despite it looking like mostly finished to be fair. Characters are fully voiced and nothing really screams placeholder content, and most updates are regarding "balancing" and mechanic changes, just trying out what's best.