When she says in BvS that a hundred years ago she walked away from mankind, that doesn't have to be interpreted in only one way. It doesn't have to mean that she walked away in 1918 for the next 100 years until the present day. It can also be reinterpreted to mean that she's recalling an event that happened 100 years ago when she walked away/ turned her back on mankind, which although for a moment, still happened 100 years ago.
This is exactly what she did in the movie when Steve was asking her to help and refused as she thought that everyone had been influenced by Ares and they didn't deserve her help. But her crisis of faith was only momentary, and in her battle with Ares she declared that it isn't about what you deserve, but about what you believe, and she believes in love.
She could be saying effectively "100 years ago I had a crisis of faith and turned my back on everyone and this is something I regretted ever since (because of the consequences of that action leading to Steve Trevor's death)." But if she doesn't give you the whole picture and say that after that happened (which was also 100 years ago) she decided that mankind was still worth saving after all, you might get the wrong impression that she has turned her back on mankind ever since.
Of course, they could just ignore that line said in BvS, but if they still wanted to keep it and not create continuity issues, reinterpreting it in this way would be a good way to go.