You are not wrong, but my understanding (and how i use mine) could also be wrong
Stremio is for streaming
Jellyfin is a self hosted media manager and relies on you having the files...if you dont have the files downloaded/stored its gonna give you nothing to display.
I set aside a day last year and set up a self host, once up and running its actually quite low maintenance. I use it for stuff i watch regular or that I want to keep (viewed and managed by JF). Also my shows because it just picks up new episodes in the background. Convenient.
For casual streaming when i dont know what i want to watch/check out a new show...its stremio.