There is a configuration "Use CDN" under site general settings is ignored when combinator is enabled. It always picks the local version and falls back to cdn only when local resource url is not set.
I see the below method from ResourceDefinitionExtensions.cs is used to get the full path which has no access to the site settings.
public static string GetFullPath(this ResourceDefinition resource)
{
if (string.IsNullOrEmpty(resource.Url)) return resource.UrlCdn;
if (resource.Url.Contains("~")) return resource.Url;
return Path.Combine(resource.BasePath, resource.Url);
}