This is a great example of deceptive marketing at work. You can check out their API here:

https://core.telegram.org/methods#working-with-messages

...where's the encryption on message histories? There is none. If the data is encrypted server-side, it's with keys that live on the servers. In other words, the encryption provides no value and the servers are "trusted." The point of end to end encryption is not to have to trust servers.