This is a very important aspect of the study. They did not do a true randomized survey as participants were self-selecting (online surveys have this inherently -- well, mostly).
It's not clear that this was or wasn't a self-selecting sample. The article does state that the surveying was conducted online:
In the research, more than 6,000 U.S. working adults over the age of 18 were surveyed online by independent research firm Edelman Intelligence between July 29 — August 24, 2016. Among them, 2,049 were freelancers and 3,953 were non-freelancers. The survey defined freelancers as individuals who have engaged in supplemental, temporary, project- or contract-based work within the past 12 months.
In the research, more than 6,000 U.S. working adults over the age of 18 were surveyed online by independent research firm Edelman Intelligence between July 29 — August 24, 2016. Among them, 2,049 were freelancers and 3,953 were non-freelancers. The survey defined freelancers as individuals who have engaged in supplemental, temporary, project- or contract-based work within the past 12 months.