To me, it represented a sort of awareness that Beatrice had towards the end. That she knew that she was losing it, and that Bojack was just consoling her, and instead of acknowledging that, she decided to just go along with it.

Whenever I hear the line, I just can’t shake how exhausted she sounds, almost like a parent trying to play along with a child’s imagination. Either way, the fact that fans are able to draw so many different but valid conclusions from a single line of dialogue goes to show how talented the writers of the show were.