Kant speaks to the "possibility of experience". Thus he speaks subjunctively: if there are rational animals, then this is what is necessary for them to have experience. They must combine spontaneous thought with receptive intuition (our direct contact with the world). Their thought must impose necessary and universal rules on the latter, rules that first have the effect of shaping apriori intuitive forms, space and time, as the conditions for receptivity (of the world). In other words, the transcendental unity of apperception that would arise would serve as the necessary and universal horizon in which any talk of reality must operate.

To imagine a humanless world as a contingent fact wouldn't alter Kant's argument. Without humans altogether, human talk of reality would disappear, and so would this thought experiment