The Supreme Court ruling grants Trump immunity for his official actions as president, but not for private actions. This amendment by Morelle is in line with President Biden’s view on the ruling, who argued that it places no limits on presidential power and effectively makes the president a king above the law.
If Biden decides to exit the race, he could release all the delegates bound to him, which would allow a new vote for new candidates at the convention in late August.
https://ace-usa.org/blog/research/research-votingrights/could-joe-biden-be-replaced-as-the-democratic-party-presidential-nominee/