I believe why Telegram is considered more secure is the source code for it's clients are open source. Which means it's possible to ascertain what the app is and isn't sending. You're not able to confidently verify that WhatsApp isn't doing more that it claims including verifying it's E2EE implementation.

Not really. It's just marketing push that 'Telegram is the most secure messenger' that somehow does the job, and the unwitting public is happy to accept it as a fact.

As of source code... do you really need a source code if you can login with just your username/password and instantly see all your chats? For anyone who has some remote understanding of the matter it clearly tells that chat history is stored on servers without meaningful encryption and that server owner can access all chat history at will.

My entire company and tg-part of a friend circle uses them when needed. E.g. to discuss issues that contain private/financial details or something personal that is undesirable to leak to "third parties who ruin parties". Otoh, I wouldn't mind if my mile-long chat with IT buddy went entirely public, because we mostly discuss Web-tech, BTC and other sick jokes there.

Personally I'd never bother to start a secret chat with a person that I don't trust enough anyway. Neither do I trust an e2e messenger that my country didn't even bother to "pucker" legally.

>users (wrongly) perceive it to be magically more secure

This is claimed often, but I'm yet to see these users. Do they really believe that, is it just tg opponent's agenda that became popular, or both?

> This is claimed often, but I'm yet to see these users

I often see these users. Most tend to just blindly trust a vendor. Also, I recall a talk at a Secure Messaging Summit 2020 given by Nikolas Unger [1], where he referred to a study about this (mis)perception.

[1]: https://claucece.github.io/Secure-Messaging-Summit/