In \Libraries\Nop.Services\Authentication\External\ExternalAuthenticationService.cs around line 150.
Why are you only checking for email activation?
I think admin approval is just as important, as I use it to only allow approved users to access the site, but I want External authentication to be automatically validated or an option on the external authentication configuration screen should allow this access.
Please take a look at the code below to see my recommended updates.

//registration is approved if validation isn't required
var registrationIsApproved = RegistrationApproved();

private bool RegistrationApproved()
{
        bool Result = false;

        if (_customerSettings.UserRegistrationType == UserRegistrationType.Standard ||
          (!_externalAuthenticationSettings.RequireEmailValidation &&
                (_customerSettings.UserRegistrationType == UserRegistrationType.EmailValidation ||
           _customerSettings.UserRegistrationType == UserRegistrationType.AdminApproval)))
  {
                Result = true;
  }

  return Result;
}