In the end, although Liwei said she'd be 'right back', she didn't show up again. As disappointed as Lan was, he shrugged it off. Maybe she was tired or unavailable to stay around him —in any case, she could do whatever felt right to her.
After eating, Lan cleaned up behind him, washed the dishes, and concocted his 'magic tea'. It was a name he came up with in hopes of recovering the initial excitement he felt about drinking it at first.
After he tasted it for the first time and realized how bitter it indeed was, he knew he would start dreading having to drink it again. The best way was to associate it with something Lan would find cool —and that thing was magic. This plant was literally magical !
In his mind, it could also be called a potion. And potions weren't supposed to taste good, but to have a desired effect.