He's saying that, in the case, they can change the narrative to cast Sanders as the choice of 'white' voters, and then - with David Brock's 'Bernie Bros' - isolate that down to 'white men.' It's specifically a strategy to divide the youth vote against itself, and one that the Clinton camp pursued pretty successfully throughout the primary - and you surely saw it echoed here relentlessly and divisively (BernieBros, Sanders does poorly with minorities, etc.).

Anecdotally, the women and minority Sanders supporters I know felt invisible, frustrated and resentful because of it. It's an obviously alienating strategy, and after splitting the youth vote, Clinton did nothing reconsolidate it, and then largely ignored the demographics she did win over with the narrative. Yes, it's strategizing to win a primary, but it's a strategy that - pretty obviously - hurt a lot in the general election, which was the actual goal. Anybody who thought that was a good idea has no place in a strategic capacity within the party.