Well that is an argument no-one is making. No one is comparing email and IM and asking which is succeeding better at being a decentralised application. You're right: email is certainly less centralised than Whatsapp. But it's a red herring. The comparison is between the original implementation of email as a decentralised system and the current experience of email, which is highly centralised.

In the olden days, any business could say in good faith "I want to offer email services" and they would have been treated pretty much the same as any other good faith provider of email service. But today, a business who wanted to start up an email service is insane; just check out the nearby thread of a person who replaced Mailchimp with a semi-self-rolled service. As a matter of fact, no matter what the RFCs say, people in the industry do not treat email as decentralised.

This is partially because people joined the network in bad faith, but it's also because a small number of large nodes are able to assume anyone who isn't known to them is acting in bad faith and therefore force people towards the larger nodes.

As I said before, email and the web continue to be usable in a decentralised fashion, but to do so involves wearing a hairshirt. This is the failure of decentralised applications. The author of the article has a view that is closer to "the market is only as free as its weaker members" rather than the view more fashionable hereabouts "the market is only as free as its stronger members", which leads to a different conclusion. It would probably lead to more insightful disagreements to discuss this underlying difference rather than arguing about what technologies email is less centralised than. (Edit to clairfy: These things in quotation marks are not quotes. The quotation marks are used to help delimit the propositions.)