Theres a tiny group of Republicans out there who hate Trump but also support any R candidate no matter what. This is the group that is changing their mind when Trump stays out of sight for a few weeks and his approval rises from 41% to 44%. These people are the intended audience for ads about an impending Bidenpocalypse. They dont want to vote for Trump, but they will anyway because its their duty as Republicans and because they think Biden might be even worse. No matter what they will never vote for a Democrat or trust the endorsement of a Democrat.

If Bolton endorsed Biden he would be a seen as a traitor and easily disregarded. Plenty of these voters are already thinking about sitting out this election, so they wont immediately reject Bolton for refusing to endorse Trump. Indifference from the inside might might soothe their fears and give some of these voters permission to stay home in November.