“Interesting observation” is double meaning here. It is also possible that Hobbes said it for himself that he observed that cliff ended but Calvin didn’t. See, even he is looking at front towards cliff, instead of looking towards Calvin.
Further, it could also mean that for Calvin, being a human, understanding inside himself is interesting (questioning existence mode).And for Hobbes, being a tiger, understanding outside himself is interesting (survival mode).
Another interpretation could be “The more inside you go, the more you are cut off from the outside world.”
That’s so nice!