The vulnerable code exists inside of the React Flight wire protocol that is used by Next.js but also Vite, Parcel, Waku and any other custom RSC implementation that exists. Your comment was accurate circa 2019 but not since React released server components.
This POC is not realistic and doesn't work against production builds of Next.js. It requires explicit exposure of gadgets by the user (eg. vm.runInContext) so is invalid as noted on https://react2shell.com
The React Server Components wire format (Flight) is relatively novel and very new (it has existed in React stable for just a year). This is not a simple JSON parsing bug.