So they do know that a transaction is valid when it is signed by the private key, then what's the point of this article? Miners don't know users private keys, there's no mechanism to move coins without private keys, to change that a hardfork would be needed, but miners don't decide on hardforks, it's up to users aka full nodes. If miners decided to make such a change, they wouldn't be mining Bitcoin, they would simply create some mutable shitcoin.

As for blocking addresses, the concept for such attack was known for a long time, anyone who is curious can find some better sources than cointelegraf.