And I don't think that would be inconsistent with Shanks character, Oda will make clear Shanks did it to prevent Loki from getting himself killed. As good as Oda portrays Shanks, he's not below doing stuff like manipulating his own family (granted they are rotten people, but they genuinely welcomed and trusted him, they weren't mistreating him like Sanji family) if he believes he's doing a service to the world and that will be the twist . Shanks did not capture Loki to punish him from killing Harald/or wreaking havoc in the new world like readers were first led to believe, but to protect him until he meets Luffy so they can team up together and free the world.
Them having an equal clash at first will be for Oda to keep ambiguous who would have won if the fight kept going normally without outright wrecking the powerscaling.