Why?

Let's compare it to F-Droid: He isn't saying that Signal should be distributed via F-Droid by default, just that there should be the option. So the article doesn't seem to say that the defaults are what matter.

But in the article he's saying not to use Signal because he doesn't consider it secure enough, and to consider Matrix as an alternative. His criticism of Signal has nothing to do with defaults, and everything with security.