The reasons are also based on the physics, which require that initial equilibration involves the rapidly equilibrating sinks in the ocean mixed layer and some terrestrial sources, while
the overall decay rate that involves slower equilibration with larger sinks is much slower.
I agree that the multimillennial «tail» of the CO2
decay trajectory is relatively unimportant in its own right, but it is not trivial, because it affects the
overall rate of
decay that includes processes that occur over many decades or a few centuries involving CO2 mixing into the deep ocean and carbonate buffering, and makes them slower than they would be otherwise.