Content from a database doesn’t seem to work in kirby 5. You can’t create new pages. (and when you add them manually in database you can’t edit/delete/… it).
The same code works in kirby 4 (except the isDraft-function which isn’t working for a long time)
I followed this tutorial:
Yes, the guide needs an update, @bastianallgeier posted a solution in another thread, let me find it 
Although, no, that was about mixed storage: Content loss on pages with virtual data - #5 by grommasdietz, just posting for reference.
I will try to come up with another example for this as well. It should be easy to adapt the storage handler to work with dbs as well. That was the entire idea around them in the first place 
Sorry for the delay. I worked on this for quite a while and decided to turn it into a blueprint. With the new changes and storage architecture, the solution for virtual pages from a database is a bit more complex, but on the other hand it also comes a lot closer to fully support all page features.
Check out the readme of the plugin for more information: GitHub - getkirby/database-storage: Adds support for pages stored in a database and let me know if you have questions.