Sure, they rolled their own encryption. But, lets look at this problem from different point of view. Every encryption protocol is invented by some group of people, Signal and Telegram is no exception, only difference is other encryption algorithms are tested, audited and verified by time for security. Keep in mind, every algorithm is rolled out by someone, you can't say there will be no better encryption algorithm in the future than all available today. So someone will roll their own crypto anyway in the future, which might be more secure than everything we have today.