That may be true for bigger artists on major labels, but for smaller independent bands it’s not always the case. I am a heavy user and fan of Bandcamp for listening to and purchasing music but I use Spotify for listening in the car and sharing playlists. I often find albums that are only available on either Spotify or Bandcamp but not both.
The ones that aren’t available on Spotify tend to be self-released but otherwise there isn’t much of a pattern. Albums not on Bandcamp, though, tend to be mediocre at best.
And that’s not even mentioning bands that are pulling their music from Spotify in protest…