This should have been named the "Völsunga saga" instead of Nibelungenlie

There are differences between the two stories despite the core main characters.

First off, in Nibelungenlied, the main character and heroine is Kriemhild, who vows to never marry because she had a dream her husband will die. Siegfried falls in love with Kriemhild before he even met her. They are soulmates. There is no love potion (or rather a "forgetfulness potion" which is used in Volsunga Saga twice, once on him and another time on the equivalent of Kriemhild, Gudrun). The story is about Kriemhild's tragic love and revenge (she marries Attila to have her brother and Hagen killed for Siegfried's death - She kills Hagen herself, before she's slain in return). Siegfried barely even knows Brünnhilde. She's just someone he has to win over for his brother in law. And Brunnhilde is more of Kriemhild's bitter rival who has Siegfried killed out of jealousy he's better than her current husband, and the deception. There wasn't any love between them. She disappears after Siegfried's death.

It just seems they were telling the Völsunga saga story (overall) but using the german version name which has a different set of lovers (Kriemhild and Siegfried, as tragic star crossed couple whose innocent love is tainted by politics and jealousy, instead of Sigurd and Brynhildr. This telling that story but using the names and title of a version where the hero and the valkyrie aren't supposed to be in love or anything like that).