The thing to know about him is he was president for a month and then he died. That was his impact. He died. He didn't noticeably improve or worsen things (Based on his inaugural speech he might've been bad but he never got to act on it) because he had no time to because he died. Which consequently means he should be the null point we can base every other president on.
If the country was left even slightly better then you got it? Then you did a better job then Harrison. Was it left worse? Then you did worse. Did 40 presidents all make the country better and only 4 leave it worse? Tough to believe.
When I read that article, my assumption was that people rated all the presidents whose name they recognized and then they just filled in the rest. Your point is valid and I agree with the idea but it requires that people know the policies and impact of each president.