Sure. But if they follow through with that threat and cost Biden the election then not only will then have zero power but they'll be less likely to be invited to the table in the future. Not to mention they will set their platform back decades.
Threaten all you want, do whatever political games you want to get your policies embraced by Biden. That's just politics as usual.
But deciding to burn the party to the ground destroys your only path to get anything done. Unless you want to take Trump up on his offer to join the GOP and push progressive policies through the right wing?