"meaning" is always socially assigned (or god's design for those you believe it).
But there is a huge difference between natural boundaries and human boundaries: one can be measured and objectively represented. You can measure heights and make a map of the earth with different colors for mountains, oceans etc. It could all be done by a machine without any human intervention, and represents useful physical differences on Earth.
On the other hand nobody could draw a map of countries' borders without talking with human societies, and no map can be objective or exact. In addition, those borders often are not even visible on the field, and in most border you can just cross and not even realize you are in another country (on roads you have border control, but most borders are just rivers, forests, fields, mountains, deserts without any remarkable feature).
But there is a huge difference between natural boundaries and human boundaries: one can be measured and objectively represented. You can measure heights and make a map of the earth with different colors for mountains, oceans etc. It could all be done by a machine without any human intervention, and represents useful physical differences on Earth.
On the other hand nobody could draw a map of countries' borders without talking with human societies, and no map can be objective or exact. In addition, those borders often are not even visible on the field, and in most border you can just cross and not even realize you are in another country (on roads you have border control, but most borders are just rivers, forests, fields, mountains, deserts without any remarkable feature).