Yes. If someone doesn't configure their username or email (using git config), git will try to create an email address using the format <local_username>@<machine_name>.local and will not set a name. If there's no user.name set, and the email isn't connected to a Github account (or isn't an email address at all), Github will display "unknown" as the commit author.
For certain repositories it's also useful to install a git hook to prevent individuals from pushing without properly configuring name/email in .gitconfig. This assumes you have administrative access to the server, of course.
