Sectumsempra has to happen for Two Hearts One Wand to happen. You cannot have the latter without the former. Sectumsempra turns Malfoid to Voldemort's cause by making her believe Harry hates her. That puts her on the astronomy tower roof, that's the reason she lets in the Death Eaters, disarms Dumbledore, and inadvertently masters the Elder Wand. If she and Harry reconcile prior to this, she doesn't do any of that. If she doesn't do any of that, Voldemort masters the Elder Wand when he kills Snape and EVERYBODY DIES. The nadir of the arc is required for it to actually reach its triumphant conclusion. And tradcons who are reinterpreting the scene as domestic abuse or a violation of the chivalric form they want the story to take are doing exactly what leftists have done for 15 years: prioritizing the story advertising their values cleanly and without exception over it actually being a good story or maintaining its internal logic.