Terrible advice. If you want e2e you can choose to enable it. It is not enabled because many users choose to receive their messages across multiple personal devices simultaneously. This is not possible with e2e, which is why it is an option.
Signal, WhatsApp, iMessage and Threema seem to do just fine.
Unrelated to this, but for all intents and purposes, iMessage cannot be considered e2e encrypted if either party has iCloud backups enabled. Apple has access to your iCloud backups, and they contain the iMessage keys.
Well, except Threema. Last time I used it, it was not possible to receive their messages across multiple devices simultaneously.
A talk on the technicals can be found here: https://www.youtube.com/watch?v=7WnwSovjYMs&t=1762s
Why not? You can encrypt a message with more than one key, no? It’s still e2e, just that there are multiple ends.
Apple’s Messages is e2e (until SMS is used) and they have group chats.