He argued that early birds, including "Archaeopteryx", lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR.
Early birds lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. This was what he argued. Early birds were including "Archaeopteryx". Early birds were incapable of WAIR. This seems. The downforce on which WAIR depends is generated by upstrokes.