It is a fact that atmospheric CO2 concentration over the course of the earths history was far greater than it is today. And if that is the case, then so were the oceans. It is a fact that the pH value of the oceans has dropped from 8.2 to 8.1. This is nothing more then a small drop in alkalinity because water does not become acidic until the pH drops under 7.0. Whether the current pH change from 8.2 to 8.1 is new and unique, or can also be greater and how it was in former times, is unknown because no long-term measurements are available.
Fallacy: false equivalence