The poll is anonymous. It doesn't show the user name (not that the user name would link to their real life anyways). There is no reason to lie, unless you think they managed to lie to themselves.I'm extremely suspicious of the responder's honesty though in those polls because 1 is a contentious subject and the other isnt.
It's like asking a group of women if they like to play tennis on weekends versus asking them if they regularly have one-night-stands on weekends. Without knowing anything about the group, the second set of responses will be much lower as there's a strong incentive not to be honest.
So I agree with your analysis with the modification of saying "But clearly, at least in the NTR fandom, players claiming they self insert is the minority by a pretty big margin."
The survey methodology is the best you're ever gonna get on the topic. I don't think there are any double blind experiments to determine what really goes inside a fapper's head.