"This survey was conducted online within the United States by Harris Poll on behalf of Blockchain Capital from October 18-20, 2017 among 2,112 U.S. adults ages 18 and older, among whom 387 are millennials (ages 18-34). This online survey is not based on a probability sample and therefore no estimate of theoretical sampling error can be calculated. "
So, sample size of 387, from a self-directed online poll, with no sampling error calculation possible. Seems legit.