They claim that criminals use Telegram, but it is not the best choice for doing crime because Telegram requires a phone number to sign up, and a phone number can be linked to identity and full location history (telcos record full location history for every number).

However, Telegram might be involved in cooperating with criminals; for example by not deleting channels related to protests against government at govt's request, by not blocking channels of allegedly spreading misinformation Western media like BBC. This is indeed illegal in Russia.