There’s a great paragraph in the article:
The people in their 20s who now make up the bulk of DSA’s membership were motivated to join the organization by Sanders, from whom they “heard the phrase ‘democratic socialist’ probably for the first time in their lives,” Riddiough said. “They have turned to DSA probably just because of the name,” and they’re now motivated to take action at the state and local levels.
They basically learned a keyword, and then searched on the web, and found the DSA, an old organization comprised mainly of 60-years-and-older people who remembered the 60s. It probably didn’t hurt that Cornel West has been a DSA member for a long time.
Bernie Sanders, however, hasn’t been a DSA member because they operate with an inside/outside strategy, to participate within the Democratic Party, and also create community pressure outside it. Bernie is an outside/outside guy, who devises his own coalitions to get elected.
These facts didn’t matter. The fact that the DSA would come up on web searches for “Democratic Socialist” mattered, a lot. It opened the door for the DSA to become the focus of a desire to join an organization. The DSA accommodated by advertising and pushing for a huge membership drive. It’s probably the most successful transfer of a left sect from the aging members to the new, young members.