What's dangerous is the framing; many E2EE messengers give the server a LOT more power than "just stores the ciphertext". https://news.ycombinator.com/item?id=33259937 is discussion of a relevant example that's gotten a lot of attention, with Matrix giving the server control over "who is in a group", which can be the whole ball game for end-to-end security.
And that's not even getting into the power of side channel information available to the server. Timing and other side channel attacks can be powerful.
Standard security practice is defense in depth, because real-world systems always have bugs and flaws, and cryptographic algorithms have a history of being eventually broken. Control over the server and access to ciphertext are definitely capabilities that, in practice, can often be combined with vulnerabilities to break secure systems.
If the people who develop the software are different from those who host the server, that's almost certainly software you can self-host. Why not mention self-hosting in the article?
If you're shopping for a third party to host a self-hostable E2EE messenger for you. The framing of the server as just "storing ciphertext" would suggest trustyworthyness of that hosting provider isn't relevant. I can't agree with that claim.