You should learn to read. He didn’t say Reagan wasn’t a great president. He said that’s not why he won re-election.
"Beating an incumbent is hard: Ronald Reagan, George W. Bush, Bill Clinton and Barack Obama won their reelections not because they were great presidents but because they were incumbents."
infers to me he feels Reagan and the others were not great Presidents in the opinion of the writer. Reagan was a great President, perhaps the greatest modern day President, and that is why he won re-election. The writer is too lazy to make the distinction and throws Reagan into the same group as the other mediocre at best list of Presidents then I am to lazy to keep reading.