He obviously isn't drawing that conclusion from "this round of democratic primaries". In a comment addressing 20 years of history, your attempt to revoke it involves looking at what one half of the parties did in the most recent few months.
[1] https://www.pewresearch.org/politics/interactives/political-...
As for Sanders. He has lost influence because he betrayed his base twice (he endorsed a candidate that the base consider to be totally corrupt) and lets face it he is too old now.
Base aside, I think Sanders isn't influential because the party insiders are strongly opposed to him. By way of contrast, despite a disastrous primary run, look at Kamala Harris.