Oxygen isotope ratio and Glaciers
It does make sense.
As you said - glaciers have lower 18O/16O. This is because they have less 18O overall. Therefore, the oceans have more 18O. But this is today.

At the Cretaceous there were no continental ice sheets, so what we said previously is not relevant. There are no ice sheets to take all of the 16O, so it's all in the ocean. This results in a lower overall 18O/16O ratio.