Rule #1 -- Beware of home made protocols
This is really Cisco showing its age. Back in the beforetimes it was easy to craft a DiY protocol for some "it seemed like a good idea at the time" purpose. (Exhibit 'A' for me would be the L2 packets Intel used to power on dormant computers -- its one of those 1990s ideas that seem really useful until it wasn't the 1990s any more.) I think people learned not to do that because it became a bit of maintenance millstone.
The actual problem sees quite prosaic -- if you can access a device on the physical network that you can control then you can craft arbitrary packets to imitate any protocol. That's hardly earth-shattering news. You'd target smaller embedded devices because they often have well known security SNAFUs you can exploit and once in its easy to program the hardware to do anything you want.