Apart from not being encrypted by default, Telegram uses its own homegrown crypto instead of a tried and tested one for its secret chat feature. That itself is a red flag.
The difference here was it was endorsed by Moxie's acquaintances from the crypto circles, followed by a very loud and aggressive disparaging campaign against Telegram led by some of these people. I've been on metzdowd list for a very long time and while cryptographers aren't the chummiest people in the slightest, there's always an underlying mutual respect. The Telegram bashing was the first time I've ever seen geniuenly vicious behavior and hate displayed towards a project that not even remotely deserve it. Almost as if the goal was just to bury the competition.
You suggest that this is the major difference between Signal and Telegram, but that's dishonest at best.