Well, the centrist Dems have been using the left-leaning Dems as expendable infantry for as long as I can remember. I recall articles three decades ago talking about how (Clinton, 1st campaign) it was the young lefties that provided all the energy into a Dem campaign, manning the phones, knocking on doors.......and then were summarily ignored when it came to policy suggestions.

For Biden, using the left, then ignoring the left is just how it's always worked.