The parties are private entities and can set whatever rules they like for selecting a nominee. That said, this was technically still the same representative democratic process. Voters selected the delegates (which are bound on the first round voting only), but Biden dropped out and released his delegates to vote whichever way they wanted.

Certainly I would’ve preferred for Biden to drop out last year and have had a full primary. But you can’t make someone accept the nomination when they don’t want it, and there are rules and a process for the already selected delegates to vote for someone else.