No, it's not the rule itself. It's rather an advice not to do as rolling own crypto is very tricky and complicated thing. You have to be very aware of many possible attacks, how they do work, to create own crypto properly

Generally a good rule, however Signal did develop their own encryption. It was so good it became the industry standard.

What does 'rolling encryption' mean (if it's possible to ELI15).

if the people you want to talk to are using telegram then you don't have much of a choice