Extraordinary claims don't require more evidence than other claims. A more accurate statement would be, "claims I'm skeptical of will require convincing evidence for me to be swayed." The "extraordinary" part makes this razor less useful, as if there were more than one category of evidence and one of them simply wasn't good enough for you.
Comey continues to advocate for backdoors in order to stop ISIS from being able to radicalize marginalized people within the US without them being able to listen. However, there are two obvious problems with this.
1. Why don't you just reach out to the marginalized yourself? Spend that $1M you paid to break into an iPhone on combatting Islamphobia and ISIS will have a tougher job.
2. What is to stop ISIS from using software written outside the US, or software they write themselves, or versions of software older than the mandated backdoors, or open-source software, or... on and on and on. His plan transparently will not work. To quote Schneier, "His problem isn't encryption, it's general purpose computers and a global market for software."