I wonder how many premium accounts Anna’s Archive had to use to scrape the whole thing. Surely Spotify has scrape protection and wouldn’t allow a single account to stream (download) millions of separate tracks.
I haven't looked at the code but I would be surprised if the premium account "requirement" is anything more than an if statement that can be commented out.
I have a feeling they didn't use premium accounts since they downloaded at 160kbit/s, which is the highest quality that free accounts can get.
Premium gets 320kbit/s (or lossless)