Let me try my hand at this and hopefully this will resolve the confusion.
Just to keep things simple. The only thing the passage is really saying as far as ice ages are concerned is the following:
Water vapor from oceans has more O16 and less O18 than vapor from seawater. The makeup of oceans (O16 vs O18) typically is unaltered because seawater vapor precipitates back into the ocean. So take any arbitrary ratio of O16 vs O18 that is evaporated from the ocean. The seawater evaporation that precipitates back should make it up.
E.g. the ocean vapour = 60% O16 and 40% O18.
It should then receive 60% O16 and 40% O18 to keep it unaltered by evaporation.
If the seawater's vapor is replenishing the ratio to keep it at 60% O16 and 40% O16 in the ocean, then when it the precipitation gets trapped as ice (ice age), the seawater just lost vapor that was at least 60% O16. Now that it has less O16 than it did before evaporation, its proportion of O18 has gone up.
Please let me know if this was clear. If not, I will try to explain in some other way!
