Sequel: a published, broadcast, or recorded work that continues the story or develops the theme of an earlier one.
Just because it tells the story from before the first piece of work does not mean it is not a sequel. Prequel sequels are everywhere and basically always have been
It’s originally used by the game company to denote “this is one of those weird things HE’s making, not our usual games” as a sort of sign of distrust from the marketing department
Meyer was just so good at what he did that it quite quickly turned into a badge of quality instead and the convention stuck