Quite frankly I don’t know, that’s the best solution I’ve found so far to this issue I explained in a previous post. I can use blueprints to create custom page creation modals that work on the content list page, but the buttons/modals don’t show/work on other pages (in my case, I want it on the edit page).
As far as I can tell, some code is missing in the edit.html.twig template that allows to display the custom page creation button & modal. It’s even actually marked as a to-do in the source 
I thought it was a simple omission, but trying it out now I realise it’s more tricky than I thought (basically the modal seems to work… but the form values are overriden by the current page values - title, route, etc - instead of respecting what I input or even the blueprint defaults…). I have no idea what’s going on tbh.
See also: Dropdown add modals not working, JS error · Issue #2259 · getgrav/grav-plugin-admin · GitHub