This phrase is so undermined, and it’s probably one of the things he says that hurts Anna the most.
When Anna met Hans, he understood her. He even loved her with all of the quirks she had. For most of her life, Anna wash’t loved for those quirks. The servants kept correcting her, and she maybe even thought that her quirks were the reason that Elsa was ignoring her. She always felt second best. But then she meets this guy who loves her and isn’t paying attention to her sister, only to her.
But he ends up betraying her. Not only does he say that he was faking his “love for her the whole time, but he is basically also saying that her sister would be a better choice.
Anna, once again, was just the spare, just the second choice.