First off, very glad you enjoyed it!
I can also answer the question re: theme as the guy who birthed the concept (also I'm the 2D artist).
The idea of invincibility was really difficult for me to work with because I am someone who very much loves playing with vulnerability in all its forms. So for me, invincibility was always going to be framed as a concept, not something literal because you wouldn't get something genuine out of me with that.
The idea of how invincibility works as a concept here is that there is no way you, or any animal in this zoo, can die. The goal of a lifelike digital zoo was to let animals actually live and not, by their philosophy, deal with the displacement and death in real zoos. A lot of it cannot be explained without outright spoiling the game, but if you want another reference of what inspired the twist, check out the story of the Deemo (2013).