"The 2018 Presidents & Executive Politics Presidential Greatness Survey was conducted online via Qualtrics from December 22, 2017 to January 16, 2018. Respondents were current and recent members of the Presidents & Executive Politics Section of the American Political Science Association, which is the foremost organization of social science experts in presidential politics. 320 respondents were invited to participate, and 170 usable responses were received, yielding a 53.1% response rate"

It's impossible to prove anything. But these people are scholars in the field, it's non-partisan, the survey methodology is transparent, the results are fairly consistent across partisan self-identification, and they are fairly consistent over time (2014 poll of same org, 2010 Siena College survey, et al). It's significant.

Trump is not a good president, don't get me wrong, but that's just not good science.

And the self-identified conservatives rated him in the bottom five, too.

Yes, they were transparent. You, on the other hand, implied equal numbers.

Nobody likes Trump. There's no need to inflate how bad he is - it's very apparent.

When you say this: "Donald Trump is objectively (ranked by presidential historians of all political parties) as the worst president in the 242 year history of the US."

That usually means that presidential historians of all parties ranked him worst, or at least that a poll controlling for party/political leaning as a variable concluded that he was worst. Neither was the case here.