Even in 2008, it was obvious Clinton was the DNC's preferred candidate and DNC still had their thumb on the scale. Despite this, Obama still managed to come out ahead. And with a base that was actually excited about their chosen candidate, they turned out to vote and Obama won.
The only lesson the DNC learned from this is that they needed to push their thumb harder on the scales the next time around.
Go look at the numbers, particularly Georgia, Wisconsin, and Arizona. Biden won those states, but by a razor thin margin. If he had lost those states, he wouldn't have won the electoral college. It would have taken less than 50,000 votes in those states flipping to Trump to change the outcome of the 2020 election.
Of course, that's just the electoral college. Biden won the popular vote by a pretty comfortable margin.