It isn't. It would be impossible for Spotify to guarantee any kind of fixed payment per stream, since customers don't pay per stream.
I guess they could pay based off the theoretical maximum a user could stream. So if the cost of a subscription is $10/month and an artist's song is 3 minutes long, then Spotify could pay ($10 / (31 * 24 * 60/3)) per stream. Thus guaranteeing that if a user streams an artist's song for the entire month, the artist gets the user's entire $10 subscription fee. But that's $0.00067 per stream, which is much less than they currently pay.
I guess they could pay based off the theoretical maximum a user could stream. So if the cost of a subscription is $10/month and an artist's song is 3 minutes long, then Spotify could pay ($10 / (31 * 24 * 60/3)) per stream. Thus guaranteeing that if a user streams an artist's song for the entire month, the artist gets the user's entire $10 subscription fee. But that's $0.00067 per stream, which is much less than they currently pay.