He makes a good point when he says that all concepts of property include the concept of abandonment as relinquishing ownership (the thing becomes unowned) and that something that is unowned can become owned through use. Everyone recognizes salvage.
The point of contention between us, I think, has to do with at what point we can say that something has really been abandoned and is, therefore, unowned.
I'll return to my favourite example of where mutualism (as I understand it so far) seems simply wrong to me. I purchase a house and land and put twenty years of development and upkeep into it. Then, I have a need to go away for a couple of years. I come back and I find that someone else has moved in and, suddenly, I no longer have a legitimate claim to that property. I've put myself into the place for twenty years and, in the short space of 48 months, I've lost it to someone who's done diddly-squat except to move in. Do you understand why that seems to me fundamentally unjust?
I suggest that two things should be taken into consideration in such a situation: 1) how much (time, money, "blood, sweat, and tears") I put into the place and 2) whether or not I made clear my intention to return.
I think it's necessary to consider what one has put of oneself into property and I think it's necessary to differentiate between intentional abandonment and leaving it for a while with the expressed intention of returning.