Eh. You raise a good and reasonable point but in this particular case I think you're comparing pomes to apples. "Web developer" is a pretty broad term, and was even moreso in 2012 when your statistic was generated. As an example: I'm a "web developer". But if I was surveyed about CSS (as a particular technology I barely understand, avoid using, and don't care about) I wouldn't respond. I would expect a number of my backend-focused colleagues to do similarly for other topics.

Being aware of sampling bias is good and useful; I'd just worry a bit about pulling too hard on that ripcord here.