This is probably some unintended leftover from the development.
In normal situations this will only cause the browser to be unable to connect. But it can actually become a security risk. The port in question (8021) is an unprivileged port, which means on standard operating systems it's possible for every user on the system to run a service on this port.
An attacker with user privileges can manipulate the webpage https://mathfacts.khanacademy.org/ for all users using the same computer.