It's also a pretty dumb article which is very surprising coming from Vitalik. 51% attacks of the sort he describes are (remotely) possible whenever a blockchain

It's also a pretty dumb article which is very surprising coming from Vitalik. 51% attacks of the sort he describes are (remotely) possible whenever a blockchain interacts with any sort of system, not just another blockchain. In his example, someone uses a bridge to create wrapped Eth on another chain, and then 51% attacks Ethereum, returning the Eth they used to themselves, while keeping the wrapped Eth on the other chain. It's just as easy to have a scenario where someone sends Eth to an exchange, sells it, and then does the same thing.