The point being made is that, if something is sensitive enough that you would hesitate to send it in the clear, you should send it over Signal or some other system that doesn't have the security flaws that encrypted email has. Given that that option exists, and that encrypted email isn't suitable for many use cases that people are tempted to use it for (and is therefore an attractive nuisance securitywise), there's not much reason to use or support encrypted email at all.