Then why did you not write 'Some places on earth are no longer fit for human habitation'?

Because that's clearly true. All of those old strip mines, and Superfund sites, and nuclear test sites, and the region around Chernobyl, and the former fishing towns of the Aral Sea are trivial examples of how sites have been rendered less fit for human habitation.

However, that logic of the argument is a bit silly, since it sets aside considering how human agency may be responsible for the removal of that part of the human ecosystem.

There really are places which "are simply not fit for human habitation" include the top of Mt. Everest, effectively all of Antarctica (the few highly subsidized settlements not withstanding), and a number of small islands (like Bishop Rock). The Marshall Islands do not fit into that category.