Background: Telegram is a secured messaging app, similar to many others (WhatsApp, Signal, ...). It emphasizes its security features and has become (like a few others) a way for people to exchange messages in a secured way. "People" can be good or bad, depending on personal interpretation.
Rival [to WhatsApp] messaging services such as Telegram are also encrypted, but their software has been written to enable law enforcement agencies to access messages where they can prove it is a necessary part of a criminal investigation.
Police in Germany knew Anis Amri was planning a suicide attack nine months before he drove a lorry into a packed Christmas market in Berlin last December, killing 12, because they had been able to access encrypted Telegram messages.
(emphasis and clarification mine)
The article is generally poorly written (the author mixes up all kind of communication channels without understanding the differences), the quoted piece, however, suggests that Telegram is not as protective of the user data they control (= stored and encrypted by them, as opposed to secret chats) as advertized.
Specifically, the FAQ for Telegram states that
The relevant decryption keys are split into parts and are never kept in the same place as the data they protect. As a result, several court orders from different jurisdictions are required to force us to give up any data.
Thanks to this structure, we can ensure that no single government or block of like-minded countries can intrude on people's privacy and freedom of expression. Telegram can be forced to give up data only if an issue is grave and universal enough to pass the scrutiny of several different legal systems around the world.
The important part is
To this day, we have disclosed 0 bytes of user data to third parties, including governments.
(again, emphasis is mine)
There is an obvious contradiction between the journalist statements and Telegram.
While the author of the article is not competent in the matter of encryption and how the messaging apps work, his mention of the specific case of the attack in Germany is worrisome. I could not find sources for his claims (and he does not cite any), only vague CNN or DW mentions of Telegram being used.
Are there any precise information on how the Telegram information was acquired?