This weakens the argument because, if the pits contain tools found nowhere else in the dwellings, then they do provide some great insight into the possessions that the people had.

This addresses the assumption that items less affected by erosion are either gone or else don't provide much insight. If scavengers took the most durable items and only the most eroded ones are left, it makes sense that the pits may not reveal much about the people's possessions.

This is irrelevant because we don’t know if removing the soil around the pits would have had an effect on the rubbish inside the pits.

The argument only focuses on the pits being used for rubbish and what that rubbish shows about the people's possessions. Previous uses of the pits are not relevant because we don't know how, if at all, they would have affected the rubbish.

The argument only addresses what the items that are in the rubbish pits reveal about the people’s possessions. While there are likely many other items that are not in the pits that also provide information, they are not relevant to the argument.