Remixes always require a license from the original creator to distribute legitimately, so Roblox’s automated detection likely stopped it from being uploaded because it sounded like an existing song in the libraries it compares uploads with
I’m not sure what sources you’re referencing but unfortunately that’s never the case; even if you completely recreate an existing song with your own sounds, the original creator still owns the rights to the composition (separately from the rights to the recording of the song) and any remix or replication of that song requires a license from whoever made it