This is my academic specialty. The reason Bruce Pascoe focusses...

This is my academic specialty. The reason Bruce Pascoe focusses on finding evidence of agriculture is because the basis of terra nullius at the time hinged upon an ability to cultivate the land. If you weren't doing that to an extensive degree then you couldn't possibly hold a defined territory down, and if you couldn't hold a monopoly on the use of force over a defined territory then you weren't a sovereign state (living in a city, for example, is only possible due to agriculture, otherwise people would have to keep moving to new spots all the time). This is still, to this day, a defining aspect of a sovereign state.

The British didn't argue there were no people, they argued there was no sovereign state. In 1788, if they claimed that and France, Spain, the Netherlands et. al. didn't dispute it then it was what it was unfortunately.