It wasn't on the page linked above but on this page [1] at the bottom it reads:
> To quickly turn Full Keyboard Access, Sticky Keys, Slow Keys, or the Accessibility Keyboard on or off using the Accessibility Shortcuts panel, press Option-Command-F5 (or if your Mac or Magic Keyboard has Touch ID, quickly press Touch ID three times).
Any other OS, you'll be able to at least tab around without the mouse.