I wish Libra was building on an Ethereum scaling technology, like Plasma, but most likely it's a new standalone proof-of-authority blockchain where the validators are the hundred companies investing $10M each.
In terms of the sliding scale between centralized and decentralized, Ethereum v1 and Bitcoin are considered to be "pretty much impossible to change history"; Ethereum v2 is expected to be "extremely impossible to change history"; but Libra would require only that the hundred companies agree to a process by which history can be changed: Libra will be very centralized in practice and while it might and I hope help take crypto mainstream, Libra is not a substitute for Ethereum's secure base layer.