I’ve just tried this on my test site, and editing discourse_narrative_bot.new_user_narrative.hello.message changed the text of the Greetings! message from Discobot when I signed up a new test user. It might be that you do have the wrong field. I think the best way to find the right field to edit is to copy a small piece of something distinctive from in the message and paste it into the search box. 
There is another similar text field for the system welcome message system_messages.welcome_user.text_body_template which might be the one?
This topic has a good overview of some of the settings and messages in the onboarding flow that may give you some hints as to which other ones you could check: