>when actually Bernie could have won if they had just stayed loyal and believed in Bernie's chances of success.

They still can. I think approval voting eliminates the need for tactical voting because each persons' vote has equal weight. A vote for DeSantis only improves his chance of winning in your case, it doesn't harm Trump, because in the end, the candidate with the most votes wins. There's no algorithm that throws away votes in a particular round, as we see in RCV.