The lesson here is complete trust in modern computing platforms is misplaced and impossible. Your hardware has backdoors, so does your OS, and encryption clients. In addition, popular apps, especially in the US, can always be commandeered by 3-letter agencies. You're only anonymous as long as you're not actively targetted, despite using "secure" apps and stuff like Tor, which media makes it seem are unbreakable.
Not quite. They were using an app developed by the police as a honeypot. Someone else had even discovered this and blogged about it[0]. If they had used email and PGP they likely wouldn't have been caught in this way. 3-letter agencies are not going to use their trump card of backdoored OS or hardware to catch drug runners. [0] https://webcache.googleusercontent.com/search?q=cache:PwQXt6...
That is because the usability of PGP is so bad, they wouldn't have any time to actually operate their criminal enterprise.
Also - email, PGP or not, leaks metadata, and the police will happily end your whole criminal career based on metadata.