Is the wiki editing restricted to employees/partners only?
Sort of.
You can email support@mikrotik.com, and ask them to register you, with your desired username and email.
You must also tell exactly what you primarily intend to do in the wiki though. If it’s a minor correction, support will do it themselves. If it’s a larger thing (like say, posting an entire page about something, or making significant updates on something), you can say what it is, and they should register you then.