Damon knows, and he knows that Elena would tell Stefan eventually anyway. They are both endlessly determined that he stay alive despite consequences to themselves. The thing is that he would do the same for either of them. Maybe he should take matters into his own hands and try to take care of it before the tag team can do so.

"You worry either way, Stefan, and then you run off to do something reckless that hurts you ultimately in the end... all in the name of trying to help me. So maybe just once I want history to not repeat itself."

The honesty comes with the decades of time in that other world and all his time in Chicago. Growth. It was decades where there wasn't much to do but to examine himself and his failings and his mistakes and his choices.