Rogue One is just another Prequel (same continuity different story thread).
Technically you’re right but they’re classifying it as a spinoff film so what can we say?
As long as the story is new and the characters are as good as the one’s in TFA I’ll be happy.
Also as long as it feels like it’s in the Star Wars universe and doesn’t clash with the OT in terms of continuity I’ll be fine with them…though I admit that the Han Solo film will be hard to sell to a guy like me.