They don't say but this would imply they have evidence of largescale fires 50K years all across the continent. If so, couldn't those have been cause by lightening or natural causes? Seems it would be difficult to locate evidence that humans actually started and created massive fires of this scale. Of course, then it's also difficult to link that to climate change. The whole thing is suspect.
The Aborigines in Australia greatly increased the frequency of fires. They used it (some still do) to flush out game and encourage plants suitable for the animals they hunted. This encouraged the growth of fire-tolerant species like Eucalyptus. In many parts of north Queensland, where manmade fire has been reduced, the Eucalyptus forest (tolerant of fire) has been replaced by rainforest (intolerant of fire).