Yeah I think Telegram, like Whatsapp, is adopted broadly enough not to bias too heavily in one direction. Definitely not 90%, and especially not if that “bias” matches the word on the street.
Also what about all the babushkas who most likely voted for Lukashenko but don't have a smartphone?
This is not to say that more people didn't vote for the opposition than the official numbers state. But Lukashenko still could have won.
There are many ways that could happen. But it's pretty good evidence to add to the context, and the pile of evidence was already quite big.
Nonetheless, I don't doubt the validity of these particular results, because I think we have enough corroborating evidence. We don't have to justify the rigorousness of a Telegram poll to come to that same conclusion.
For instance people in the US still use text-messaging to a significant percentage. That's hard for me to believe as well.
It's no use looking at Telegram's world usage, that's for sure.