I took this to mean that the people who are currently supporting Sanders are the ones currently most fired up for change and contain a substantial number of the Democratic voters at this point (see also: AOC’s supporters). The fear is that if the DNC go business-as-usual and repeats the mistakes of the 2016 election - namely alienating those supporters - it will cause exactly the “it is so easy to say the hell with it, a pox on all of them” attitude mentioned in the article.
The issue isn’t this supposed “Bernie Bros” bloc leaving, it’s that the DNC constantly is moving politically to the right, and actual liberals are giving up. Why support a party that constantly fails to support you and your interests? Why send money to a party that marginalizes and abandons candidates you like? Why support a party that uses internal politics to suppress a candidate that actually fires up voters (and young voters at that) in order to nominate a political insider that was one of the most hated candidates of all time (rightly or wrongly is beside the point - her public image was toxic, and she should never have been nominated).
