Disable site visiting and user logins excepting for a specific user

Your if case matches any user who is not both a super admin and test_user.

For example, given a user test_user who is not superadmin, the result would be

if (true || false) redirect

Since one condition is true, the redirect would be triggered. Using the && operator, you’d get

if (true && false) redirect.

Since both are not true, it doesn’t trigger the redirect.