It seems to me that this passage is conflating two distinct issues:
1) whether your decision not to purchase one pound of chicken will shift the demand curve for chicken by one pound i.e. whether it will cause producers to know that there is now one fewer pound of demand for chicken at the current price
2) how much that shift in the demand curve will change the quantity produced i.e. how much the production of chicken will decrease given that producers will lower the price of chicken and that the demand for chicken will be greater at that lower price (thus partially offsetting the reduction in demand that you caused)
The argument that MacAskill makes is related to the first issue, but the evidence he cites from Compassion for the Pound is related to the second issue. I think MacAskill is correct on the first issue, but I do not think that the evidence he cites supports his position.
Is there other evidence supporting or falsifying Will’s position from another source.