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
I can't think of another example where a title with a 2 in it had a story that took place before the first one. The closest I can think of is The Godfather 2 (assuming I'm even remembering right) and that had the prequel parts as flashbacks within a story that took place after the first one.
Kind of an odd one there in that as far as I understand, they were reusing one of their IP's, Red Dead Revolver.
Ironically, I played Red Dead Revolver as a kid, never played RDR2. That said, it's not like it's a cohesive universe between the two by any means, so your point still stands.
There's Red Dead Revolver, Red Dead Redemption and Red Dead Redemption 2. Revolver is thematically similar but beyond that isn't related to the other two. RDR and RDR2 are continuing the same story but the events of RDR2 take place before RDR.