What Singh is implying to coders is that cryptographic messages of high importance should be done well. For instance, if the contents of the message are a correspondence between about a politician having an affair then; it would make sense for the code to be very strong so the politician’s job isn’t jeopardized.
On the other hand, if it is a playful message being sent between friends with no real significance then the consequences of it being coded are not drastic. It doesn’t make a difference how strong the code is. If the stakes of being caught are high, then make a strongly coded message that nobody will figure out.
Singh means that if instead of a message being poorly coded and it was just straight up then it would be more of a; “Yeah, this is hard evidence against you and there’s no denying it.” Then it would be used and that would be that. But because it was supposed to be hidden it adds an extra layer of distrust to the case and provides further justification for conviction. In Mary Queen of Scots situation, she was doing the most incriminating offense possible towards the Crown and the fact that she made a code to hide her plans hurts her legacy in the end.