Earlier quoted context omitted.
> Starting from these as axioms, what can we construct? Nothing, unless there is also a physical world which actually exists independent of any of the participants' will.
This text is in the physical world. This statement exists on your screen independent of my will. It may exist in the first place because of my will, but its continued existence before you right now is independent of me. We have a shared reality in the form of this very text based medium.
But I am only convinced of this fact because I have experienced it independently from any other agent. I know for sure I exist in some sense, I believe very strongly that the physical world exists, and I believe to a lesser degree that other agents have similar experiences to mine.
In other words, I am more certain of the fact that your comment exists than I am of the fact that your subjective experience exists. If I found out the comment I am responding to was in fact GPT-3 output, I would be much less shocked than if I found out my own senses or memory deceived me.
So, the physical world is a much more believable explanation than a world of conscious discussion; and exploring the physical world in the aspects we ourselves can observe about it alone is a much more convincing argument than trying to discuss it with other conscious agents.
It's much easier to convince someone else that I can pass through walls than it is for me to actually pass through a wall.