On the screencast it appear that the dialog box have been reworked to be less constraining than in Sierra where you needed to go to Preferences>Gatekeeper to allow execution of unsigned app.

I haven't tested the beta so this struck me, can someone confirm?

Right-clicking an app and selecting "Open" has always given you the ability to bypass Gatekeeper. It's been that way since Gatekeeper was introduced.

I just downloaded an unsigned app (https://www.bitcoinarmory.com/download/), and I see the usual `"Armory" can't be opened because it is from an unidentified developer.` message, with only an [OK] button to cancel the launch. I see that the video maker disabled Gatekeeper by choosing Open from the right-click menu.

The app is opened by right clicking and picking 'Open' from the context menu, this hasn't changed.