WOW!
I turned on the demo - and got stuck myself!
What a pain in the neck.
But - you can get out of it.
Draw an L on your screen, a generous one - down then right, per -
https://support.google.com/accessibility/android/answer/6151827
This gives you a "global context menu" -
https://support.google.com/accessibility/android/answer/6007066
When I did that, I got two semi-circles in the upper corners - one was to pause the demo, mine was in the upper left - choose that.
You may have to double-tap it, but start with one tap. If it throws a dialog asking if you want to pause with an ok, try double-tapping the ok - also try tapping the OK with two fingers at once.
That should get you out temporarily.
Now - settings, Accessibility, Talkback - off!
Hope this helps - I don't have your phone but this feature is supposed to be somewhat standardized. So, either my steps, or those links and a little trial and error will get you there.