These metrics rely almost exclusively on temperature data from the Argo profiling float array.
I wish that were so! Unfortunately, there were no Argo floats in the 20th century, and NCEI OHC estimates go back to 1955!
https://www.ncei.noaa.gov/access/global-ocean-heat-content/index.html
For 2/3 of the time period for which they claim to know OHC there's almost no measurement data from which it can be determined. It's models, all the way down.