That report says:
a lot of people self-selected as both a web developer and designer
While the SO FAQ says:
If your question is about … Web design and HTML/CSS layout, and your job title is "designer", ask on Doctype.
So the answer to your question:
I was wondering if some policies … are contributing towards the negative image of Stack Overflow among web developers.
is definitely "yes."
If "a lot of people" who answered the survey think of themselves (at least partly) as designers, and the SO FAQ tells Web designers that their HTML/CSS/Layout questions are unwelcome not just on SO, but on any SE site, then it's not really surprising if SO isn't better loved by that group.
Yes, I know that there's no shortage of HTML, CSS, and even Web design questions—but they're from people who either don't read FAQs or don't care. Is that really who we want to favor?
My preferred solution, btw, would be to see any of these currently existing Area51 proposals take off: Web Development & Design, Web Design, and/or CSS Expertise.