Hello,
WebEx API uses a different codebase than the base product. In many cases, XML API uses an old hardcoded template for email notifications, rather than site email templates, and does not include the ICS attachment. This is actually an open feature request, however, but the XML API is no longer being actively managed and is primarily in maintenance mode. The original API design intent was to hide the WebEx product behind a developer's application or service, relying on the developer to manage email notifications through their own email engine, and giving greater control over the content and origin of email notifications.