Under Manage Project Properties, Artifact Types in DNG v. 6 there only seems to be "Diagram" as default artifact format available for graphical artifacts - does this mean that artifacts like storyboards and screen flows must be created in the new diagram editor?

The documentation only states:

Tip: You can now create screen flows [and storyboards] using the new diagram editor, which is the preferred tool for creating graphical artifacts.

It is not stated that other tools are available.