The reason for this conclusion is that, unlike the open ocean, pCO2 in coastal ecosystems is not necessarily in equilibrium with the atmosphere at even annual timescales and many coastal ecosystems emit CO2 into the atmosphere (Laruelle et al. 2010; Cai 2011). (link.springer.com)
Winstrup, M. et al. (2017), A 2700 - year annual timescale and accumulation history for an ice core from Roosevelt Island, West Antarctica, Clim. (usap-dc.org)
An additional ploy seems to be the implicit assumption that temperature only depends on CO2 concentrations, even on annual timescales. (realclimate.org)