The study looked at almost 40 years worth of observational data from multiple correlated sources. That is one of its contributions relative to similar studies that looked at fewer sources of measurements, or used different methods.
I am skeptical of your claim.
15.9 million square kilometers was the recorded antarctic sea ice coverage in August 1966. https://ntrs.nasa.gov/api/citations/20140017193/downloads/20...
You seem to imply that short term year on year variations in Antarctic sea ice (between 1964 and 1966) is a reason to doubt the multi-decade measurement of gulf stream weakening discussed in the article, but you have provided no reasoning to support your opinion.
1979 was the maximum, and nobody disputes that.