He was just referring to the consensus of historians. He’s not wrong. Buchanan is either 1 or 2 in all of these (and thus worst overall):
”In compiling its 10 Worst Presidents rankings, U.S. News averaged presidents' scores from three separate metrics: C-SPAN's 2017 Presidential Historians Survey, Siena College's Presidential Expert Poll and the Presidential Greatness Rankings conducted by professors at the University of Houston and Boise State University.
Those narrowly missing the cut were Presidents Zachary Taylor and George W. Bush, who tied for 11th-worst among the nation's 44 commanders in chief. President Richard Nixon, who had previously ranked among America's 10 worst presidents in previous iterations of the U.S. News rankings, was remembered more fondly by recent presidential surveys. He ranked 15th-worst, just ahead of Martin Van Buren and Calvin Coolidge.
Ulysses S. Grant made the biggest jump since 2014, when U.S. News last updated its Worst Presidents methodology. Grant was considered the seventh-worst president in U.S. history just a few years ago. He's now considered historically average, ranking 22nd-best out of the 43 presidents that were considered.
U.S. News opted not to include President Donald Trump, who has yet to finish his first term. But he was deemed to be the worst president on the Presidential Greatness Rankings and third-worst in Siena's Presidential Expert Poll.“
https://www.google.com/amp/s/www.usnews.com/news/special-reports/the-worst-presidents/articles/ranking-americas-worst-presidents%3fcontext=amp