I'm not sure. Maybe that's the case for the passwords which can be changed via the administrative app but I read many of these are in firmware and not able to be disabled or changed:
“The issue with these particular devices is that a user cannot feasibly change this password,” Flashpoint’s Zach Wikholm told KrebsOnSecurity. “The password is hardcoded into the firmware, and the tools necessary to disable it are not present.
- https://krebsonsecurity.com/2016/10/hacked-cameras-dvrs-powe...
That's not to say it couldn't flash the devices but I don't recall seeing that capability in the Mirai source and haven't read about it doing so.