If the chat isn't e2e, you are warned at every moment because the message text box that you type into prominently says "(unencrypted)". You can easily enable e2e for all conversations, and it will soon be the default. The data is on your own homeserver, and no one else has access to keys. Riot/matrix is vastly superior to telegram.

Edit: also, telegram doesn't support e2e encrypted group chats, unlike riot.

One of them does rather dishonest marketing, the other is upfront about capabilities and defaults.

Telegram: we do E2E!* That makes us MORE SECURE than other messengers!

Matrix: you can enable E2E if you want.

* just not by default†

† unless you count "E2E" security between your client and our servers,‡ which we'll confusingly highlight in our security documentation as if it were special††

‡ you don't really need E2E anyway because we store your data in shell companies across the world

†† I guess it is "special" because Telegram invented its own crypto algorithms?

I'd like Telegram a lot more if they would stop with their bullshit claims, because it's actually a good service for some usecases. While reading the article I was hoping it was just written by a clueless Telegram fanboy, but reality is disappointing…