It's not an uncommon sentiment. The Sanders campaign has some great databases and MoveOn.org started out as a petition to get Congress to drop the Lewinsky scandal. The cynicals among us have zero belief that Bernie Sanders ran for president with the intention of winning the nomination. It would be entirely in character for him to have run with the express purpose of hectoring Clinton into moving left, which she's been forced to do.
That's not even cynical, it's giving him credit for not being completely naive after so long in politics. There's no way he could win, and he surely knew there was no way he could win, and it's much more useful to mobilize an actual left in mainstream politics than to be a socialist President when the current state of American politics places the left at "Reagan", moderate at "the John Birch Society" and right at "herp derp build a wall to keep the brown people out."