The main reason I consider it suspicious is that they are so adamant about not needing end-to-end encryption.
Even assuming they are fully legitimate today, if this ever changes and somebody gets access to their infrastructure, they immediately get a treasure trove of historical messages.
Telegram hoards messages of over 1,000,000,000 users as of now. Anyone not realizing major nation states have long since pwned the servers to spy on those users is a fool. Even if if the zero days cost 100 BILLION dollars, that access costs like 1-2 hours of intelligence contractor's hourly wage per target. The bang for buck is insane. Telegram is one of the world's biggest intelligence operations, wittingly, or unwittingly.