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.
Given that polar sea ice peaked in 1979, but decades before had minimums below anything recorded since, and this study begins with 1982 data, I don't see how they can claim such certainty.
That link is a paper about Antarctic winter sea ice extent maxima, when your claim was about polar sea ice minima, but I did not follow your argument in any case.
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.
Okay... Winter minima is an important metric cited by "climate scientists" (and reporters) far and wide. Summer minima is less interesting for a whole bunch of technical reasons that I'm probably not qualified to explain.
