I think the author is confused.  Millennial aren't backing Bernie because of his 'socialist' title.  They're backing him because he articulates and opposes the nearly complete control of our government by corporate and extremely wealthy donors.  

Millennial have come of age seeing a highly dysfunctional and corrupt government that's failing to meet their needs and effectively address serious issues, focusing on war and wedges instead.  I don't think their opinion is going to change as they get older unless our government fundamentally changes.  The winds of change are coming.