The documentation is bad there. You need to copy it into your inventory/${CLUSTER}/group_vars, and then it will be picked up correctly.
cp inventory/sample/group_vars/no-floating.yml inventory/$CLUSTER/group_vars/
It is terribly annoying that it is hardcoded to modifying sample, rather than just assuming that ${var.cluster_name} will be a reasonable guess. Not only is it confusing for the reason you state, it is also confusing from a version control point of view.