I don't know specifically about Cardano but this news does not surprise me.
My understanding is that the dream to develop a "Proof of Stake" cryptocurrency, got a lot of setbacks and it seems that in practise is not possible to give up "Proof of Work" if you want to remain secure and decentralized.
Even the upgrade plans for Ethereum abandoned the full "Proof of Stake" idea, and switched to a model that uses a mix of PoW and PoS.