Hey there,
Thanks for your message. The sheet will not be separated since it is used by thousands of devs this way. Please load the TMX file that is included if you have trouble using it. It is an example for exactly that usecase. The biome tiles all have dedicated areas in the PNG which should be quite easy to see when loading the TMX in for example tiled. They are intended to be used together; mix and match. That's why it's one file: also because people had exactly the opposite issue with multiples.
Kind regards,
Sam