Adaptability is certainly not necessary (almost by definition) if you're playing a near to equilibrium strategy but adaptability is a useful skill to have in a general non-stationary world.

That said, for this bot, I wouldn't say it's playing completely independent of the other players's interior state. Pluribus must infer its opponents strategy profile and according to the paper, maintains a distribution over possible hole cards and updates its belief according to observed actions. This is part of playing in a minimally exploitable way in such a large space for an imperfect information game.