This article glosses over a few important details.

The reason that end to end encryption exists is precisely because man in the middle attacks are possible, and always will be. This kind of attack is devastating for Telegram, because by default they don't use end to end encryption, and instead store the entire plaintext history of every message you've ever sent or received, all of which an attacker instantly gets access to.

For WhatsApp, everything is end to end encrypted by default, so the attacker doesn't get any message history. All of the contacts for the MITM'd user also get a notice that their contact's security code changed, and a comparison will fail to match. This is exactly what E2E was built to protect against.