That article performs some sleight of hand, where nodes "control" the currency. Nodes sign transactions publicly, they don't control anything. They can't covertly pick the wrong transaction to sign, if they do it will be very visible.

I mean, I may be wrong, but don't consensus between miners determine whether a block is valid or not? Specifically, 51% of mining power determines this, AFAIK.

Yes, but you can verify blocks independently and see that someone verified a block that was actually invalid. It's not something that can remain secret.

But you won't verify the blockchain and call every node to say "hey! big corp X is cheating". The consensus protocol is what it is and if 51% agree on anything it will go on. We are at the mercy of 4 giants messing with the chain. Also probably simple double spending could be noticed and proven wrong but, for instance, rolling back a transaction would still be possible breaking the immutability of the blockchain.