It’s really not. Just as the development of a corporate brand or a consumer brand — the product/service, the environment, the communications and marketing, and the people delivering the brand experience — has nothing to do (from a development perspective) with the ads you see in newspapers or on television.
It’s the strategy. It’s the foundation. The playbook. You don’t need to know how to make a television commercial or code a banner ad or design a print ad to do it — although of course that helps (quite a bit). You do need to be able to get under the skin of the company, or product, and discover its underlying essence (or DNA or whatever) and align that with its vision, strategy and values.
On the other hand, it has everthing to do with attraction, engagement and retention. You need to know job seekers as audiences, and know your employees. You need to know where and how to interact with them.
The point is, there is no reason why you wouldn’t have a brand or strategic communication agency (or an MBA) work on your employer brand and then handover to those who buy ad space for you in print, TV and online. Two completely different skillsets, although some would like you to believe differently.