Statistically speaking 4000 is a good enough number. I mean, election polls in the US usually use less than that for a population of 300 million and they're usually close to the real number.
In 2016 the national polls were very close to the final result--within a couple points. Remember that Clinton won the popular vote.
State polls were less accurate, but the general consensus is they were wrong due to undecided voters breaking more for Trump than is normal in the final few days--after the final state polls were conducted.
That being said, your point stands. Sample method is generally more important than sample size, and 4,000 game devs who self selected to attend a conference is very unlikely to be a representative sample.
It seems to me the GP comment meant that because it was a survey among people who attended GDC it's not representative. If that is why they questioned it, I'd say it's an even more relevant survey, as those are the people who care more about the industry and have more influence. No mater how you slice it, I'd say it's an indicator not to be ignored.