Maybe you're not sure how words work. 'Universal' means 100%.
Biden doesn't support universal healthcare. Neither does the Democratic party at large. They support the ACA because it benefits their corporate donors in the medical and pharmaceutical industries. The ACA doesn't cover everybody, and many that it does cover it remains too expensive for, and not affordable at all, while 45K-68K people continue to go bankrupt due to medical costs every year. As the result of this CA-25 special election has shown, most voters aren't enthusiastic about voting for someone who doesn't offer them anything. Christy Smith's positions on policy weren't all that different from Katie Hill's (and Hill endorsed her), except when it came to her position on healthcare and establishment influence.