The skew is something they're supposed to take into account, if they realize a given factor is important. Models are seldom perfect, and people can be incompetent or self-centered even when they're not being actively malicious. I think you're overestimating both the quality of the statistical models and the purity of the motives behind this survey.

The problem is that this assumes a perfectly normal distribution. There's a possibility that anime fandom likely doesn't follow a Gaussian distribution (dare I say it's almost certainly not Gaussian, because anecdotally, people seem to either be neck deep into it, or are disgusted by it, with nary anything in between). If this is true, the above calculator doesn't exactly work.

I mean, I would hope that a company that exists to survey people would be competent in said field. But as someone who has a diploma in maths, I'll say that I do think four thousand people simply isn't enough.