Why would it not be believable? Right after that Ellie goes question him why he were still trying to protect here when she said she didn't want it.
Why? Because she knew he loved her more than the whole world, that's why she was angry, she was confused and conflicted between he denying her life to be meaningful and knowing he did it out of love.
When she goes confront him after that it's very clear, again, she even tells him she don't think she can forgive him and still want to give it a try because she knows both love each other.
And that's the one of the biggest reasons she doesn't kill Abby in the end, she is still traumatizated by she brutally killing him, but she knows well he didn't want hate to consumer her, neither did she, that's not who she is, never was, as Abby was now freed from revenge herself she changed like Joel, now she was Joel for Levi, and Ellie didn't want that cycle to keep on.
The amount of connections in every single piece of their characters and story is very well constructed and laid to the player during the game, it's not hard to see, at all.
I was played by the game from being disgustedly mad to have to play with Abby for some time and by the time I was finishing the game I was saying in real life for Ellie to not kill Abby when I didn't know she would let her go, I have only to applaud to ND team on every level for the story they developed with this game.