The planet absolutely could not support 8.5 billion people engaging in subsistence level farming, or hunter-gatherer living. At all.
The proof of this is North Korea, which is broadly in the middle of major ecological collapse because it's farming doesn't work and the people have essentially stripped the land bear trying to gather enough food to survive.
Wikipedia says there are more than a hundred billion wild mammals. [1] They obviously do not all consume as much food as humans do and there are certainly a lot of small mammals like mice, but from the order of magnitude it seems at least not completely obvious that ten billion people would be too much to support without fertilizers other than dung and similar things.
From a different angle, Wikipedia says there are about 1.4 billion hectares of arable land and the best number I could quickly find is that one needs one to two hectares per person without modern fertilizers, that would imply a limit of only about a billion people.
Agreed: tractors and other farm machines are a tremendous productivity multiplier. Nitrogen fertilizer, which the world currently makes from natural gas and would require advanced industrial infrastructure no matter how it is made, is another.
This neglects that Permacultural farming is beneficial for soil and feeds way more people per unit of land, but requires more labour than monoculture trowel and row farming.
The proof of this is North Korea, which is broadly in the middle of major ecological collapse because it's farming doesn't work and the people have essentially stripped the land bear trying to gather enough food to survive.