Web3 is a highly ambigious term so there's no point discussing whether it's decentralized - it simply depends on what system you are referring to.
For the sake of simplicity we stick to the authors main claim. That Ethereum is centralized.
> while anyone can join the Ethereum or Bitcoin network, you can only join if you agree to follow the same protocol
In which other protocol is this not the case? Can you communicate with a web page without using the protocol that it required?
Furthermore, as blockchain protocols are all open source, they allow their network rules and data to be easily forked and run. This is not at all dissimilar to an example provided further that explains that anyone can run their own web network.
> Ethereum is only decentralized in the way that doesn't matter — you're free to join the decentralized system, under the condition that you act in the exact same way as every other actor in that system.
The opposite in fact. The author misses that Ethereum allows anyone to encode their own protocol into the network. Blockchain nodes, simply enforce that those rules are being adhered to by the participants. This is perhaps the only way that matters for what etheruem wants to do. Blockchains are not websites, their purpose is not to serve you data. The purpose is, given a pre existing set of rules - which anyone can code permanently into the blockchain, clients understand exactly in what way those states can transition, and are capable of executing the state transaction as equally as all other peers.
Let's take a practical example. Say you want to publish torrent links. I hope I don't need to explain why dencetralisation matters and what propoerties you are looking for. If someone encodes a smart contract on Ethereum that allows anyone to submit a name, along with a torrent link but not able to delete it. In the Ethereum world, this protocol lives forever more. On the traditional web world, you will:
- Have ISP, hosting provider take your website down
- The web owner may decide to arbitrarily take down specific torrents or the application as a whole
This is where decentralisation matters and what it means.