People make bootleg routers (internet routers), modules, line cards, transceivers, carbeurators, fuel pumps, injectors, phones, stereo equipment, circuit breakers, dental drills, and so on. Its not just clothes and purses. Some are gray market with some real parts from the same factory, some made from scratch. If it's a well known brand and money can be made, it is at risk of being copied. Some end up at trusted retailers because they were injected at the wholesaler/distributor.

That being said.. I have no idea about Shimano or this der. Not much concrete evidence comes up with fake Shimano derailuers, a bunch of people asking though. Just my opinion like others mentioned, derailuers are not high volume, mainstream, and not really that expensive or artificially priced too high so not really easy to undercut.