In reply to Andrew.

If the onboarding was in a plugin, it could be designed in such a way that the user experience would be exactly the same.
As it is now with the onboarding included in the theme, the user activates the Ollie theme and they are presented with a notification inviting them to click a button that takes them to the onboarding page.
This could be exactly the same process for the user if the onboarding code was in a plugin. The same theme notification could be presented, with the only difference being that when clicking the button the plugin would be installed and activated (this is allowed because themes can recommend plugins), with the plugin then providing the code to take the user on to the onboarding page.
There would be no noticable difference in the process as far as the user is concerned.