This is a nice extensive writeup.
Unless it's doing it by default now (it wasn't when I last checked a hundred million years ago) you need to specifically enable to eend to end encryption for Telegram per chat.
Nice to see options now too, I remember when I was starting to bail out (haven't completely bailed out yet, keep having to enable Google scripts on various websites including peakd so they actually run >_<) and with what I was doing at the time and the options available I ended up rolling my own mailserver (that was hairy x_x).
Yes, Telegram has written extensively about the trade-off between end-server-end encryption and end-end encryption. The former is more convenient and allows conversations to continue across multiple devices. The later more secure.
They opted to provide both options with the former the default.