Not strictly correct. Spotify pays out from its net revenues (revenues when billing costs and tax are removed) and it pays to the various industry rights holders who then distribute the money. There are lots of complex deals in place and big rights holders are likely to have better deals than ad hoc users, plus it's different in different countries.

The 70% figure is a PR thing Spotify pushes about as part of its constant battles with rights holders on exactly how much it will pay them. It's trying to claim most of the money goes to artists but it's opaque how much goes where.