https://en.m.wikipedia.org/wiki/Big_Five_personality_traits
https://en.m.wikipedia.org/wiki/Agreeableness
It doesn't seem controversial to say that if a survey is presented relatively apolitically, someone who scores higher on agreeableness is more likely to be willing to participate in it.
This is the part that seems dubious to me. One, nothing is apolitical anymore. Two, why is it given that people who score higher on agreeableness more likely to answer surveys? If the internet has taught us anything, it's that it's far easier to rile up responses from people who disagree with you.
This survey wasn't brigaded because people were selected at random across various demographics. There was no direct visual feedback of what others thought. It was done online, so you couldn't even get feedback from the person conducting the survey.