Pretty much everyone voting in November is either voting for Trump or against Trump. Thus, I suspect that the DNC Polling showed that the more choices that the anti-Trump vote gets, the more those voters will split their vote. The only way they could have helped Biden was to get a “Trump without the Baggage” candidate, and they simply don’t exist*.
*DeSantis was thought to be that person, but he too bowed down to the Neocons and thereby collapsed as a candidate.