Thanks for creating this post and thanks to the knowledgeable people who have responded.
I agree with adding a warning or some kind of notification.
Like you, I got this error message after (re)installing NixOS. After rebooting, however, my /boot directory is correctly owned by root. If I’m following this thread correctly, these corrected permissions mean that there’s not a persistent security hole.
I’m sure others like us will see this error message and panic a little at first until they find this thread or until an explanation is added to the installation process. 
So adding that notification is a great idea.