The technology was already proven, i.e. The Pirate Bay and other torrent networks had already been a success for years. What Spotify likely aimed to show was that they could grow very fast and that their growth was too good to just shut down, like the entertainment industry tried to do with TPB.
After they took in the entertainment oligarchs they cut out the warez and substituted with licensed material.
Always made me chuckle, it looked like they had copied half of their catalogue from the pirate bay. It took them a few years to clean that up.
IIRC, 2008, a little less than twenty years.
Spotify showed that you could have a local-like experience with something backed by the cloud. BitTorrent had never really done that. The client wasn't that good, and you couldn't double click and hear a song in two seconds.
The way you said that made me think you might be remembering when it was partially P2P, but I don't remember the timeline, it was only used to save bandwidth costs, and they eventually dropped it because network operators didn't like it and CDNs became a thing.
Ek had been the CEO of µTorrent and they hired a person who had done research on Torrent technology at KTH RIT to help with the implementation. It was a proven technology that required relatively small adaptations.
They moved away from this architecture after the entertainment industry got involved. Sure, it was a cost issue until this point, but it also turned into a telemetry issue afterwards.
