Hi there,
This was patched by us months ago, long before the PoC you see here was released according to the researchers who developed this. We don't keep older versions of GrapheneOS to know how functional it actually was, but for simplicity we'll act on the assumption the entire exploit chain worked.
As it stands you need both a vulnerable version of an OS and Firefox (which they link as Firefox also patched) to do this. You should never install outdated applications, and as a plus we tell people in our usage guide to not use Firefox because the security of the mobile browser is poor.