> How did fish manage during the climatic optimum [1] some 8500 to 5500 years ago when it was between 1.5°C and 4°C warmer than it is now
It was not 1.5—4 °C warmer. Mean air temperatures around the peak of the HCO were less than 1 °C elevated relative to preindustrial temperatures and were below last decade's mean air temperature. And those aren't ocean temperatures. Regionally, surface waters around the great barrier Reef was ~1 °C elevated relative to modern day.
Anyway, the answer to your question is the rate of change since was much slower than recent temperature /pH changes. Populations, even sessile ones like corals can more successfully redistribute and move toward the poles to find optimal thermal conditions with a slower rate of change. And we are also putting the ocean under a few other confounding stresses like overfishing, eutrophication, acidification, coastal development and habitat loss, etc, which is novel territory relative to previous climactic shifts.
Also I'll note what might be a blip in the geological record in terms of fossil richness could still have profound consequences for species on which we depend on for economic livelihood and food security, e.g. say you have a well managed fishery harvested at close to maximum sustainable yield. If conditions change such that the environment rapidly shifts to support only 80% of the original carrying capacity due to growing OMZs, the fishery now needs to be managed such that it will support a lot less jobs and produces less food (or, mgmt doesn't adapt and the stock gets severely overharvested, and then the job and production loss necessarily follow).
> I suspect this balance between higher atmospheric CO₂ which leads to an increase in vegetation which then leads to an increase in oxygen production is what kept ocean life from succumbing due to oxygen deprivation.
The article mentions why this suspicion is wrong. In fact, proliferation of the main oxygen producing taxa in the ocean is one of the primary mechanisms that exacerbates oxygen minimum zones. Phytoplankton blooms, then dies, sinks, and get eaten by microbes that consume oxygen.