Bojack Horseman was the only character who saw Vincent Adultman as three children in a trenchcoat pretending to be an adult. All other characters seem to have taken his adulthood at face value despite his (their?) appearance obviously matching Bojack’s evaluation.

This could be taken as a commentary on the self-absorbedness of Hollywoo’s inhabitants. In most cases, when they thought about others at all, they saw them only as extensions of themselves, identical to their own mental simulations thereof– i.e. exactly as they expected to see them.

Thus, Princess Caroline wanted a romantic partner and expected that she could find one, so she saw Adultman as someone who could be that for her. Todd and Mr. Peanutbutter, who tended to follow their own wild schemes as if swept along in a current, found Adultman along the way and accepted him as being whatever type of consultant they needed at that point in the adventure. The only character unlikely to do the same was probably Diane, which may be why she was never given a storyline interacting with Adultman.

Bojack himself had a worldview that often convincingly mimicked realism, by virtue of being extremely pessimistic and cynical in a world where the reality usually matched that. Which is not to be confused with an actual aptitude at seeing reality as it is.

Bojack, I think, saw the deception of three children in a trenchcoat not because it was the reality, but simply because he saw most people as being phony and deceptive by default, which happened to be correct often enough that the worldview more or less served his needs.

The question of whether it did in fact happen to be correct in the case of Adultman– or whether Adultman really was a perfectly normal adult whose appearance is filtered to the show’s audience through the lens of Bojack’s perception – is an exercise left to the reader.