I think this may just be a matter of changing frame of reference. If you want to say chaozhou and shenzhen are far away its not wrong. If you want to say they are quite close, its not wrong either, here is a map for easy visual ((courtesy wikipedia)). I chose one with basic language mapping for also convenient reference, although not directly relevant to the answer:
Its about 180 miles//290km between these cities, thats a few hours by car-- or there are almost one hundred high speed trains every day between them, each ride only 2 something hours. It wouldn't be recommended at all, but theoretically you could even do it as a daily commute!
Thats distance addressed, now why would people flow from chaozhou to shenzhen? also a bit straight forward with right frame of mind; shenzhen has been an economic hot spot for a long time, and in fact was specifically created and developed to be one, it is one of the fastest growing cities in china ((and the world)). So people from all over flow into shenzhen, not just chaozhou.
Now, why people from chaozhou seem to have so many in one career path in shenzhen? you guessed it, lets lay out a few perspectives-- I don't know exactly which one applies here, but can show a few examples of how this can logically happen.
example one, lets say there is a very good tech school in chaozhou, that specializes in hardware mechanics. lots of people graduate, many people flow to bigger cities, and many of them go to companies in shenzhen.
example two, lets say there is no college involved, but a few people from chaozhou start that business in shenzhen. well, they invite their friends or family to come too, work for or with them, teach them the business. Some stranger or friend of a friend hears about their success, decides to go there too, or maybe even their enemy decides to go there and be better than them-- thats silly dramatic, but you get the idea.
example three, lets say one super successful business is in shenzhen in that category, anyone in that same line of business could decide yo go and try it for themselves, get a slice of the pie. its not actually required to have a direct connection at all "if they can do it, I can do it too" is a very powerful mentality.
again emphasis is that shenzhen might lack anything else you need, the one thing it won't lack is potential and opportunity. just like other mega business scapes like new york city, hongkong, tokyo, wherever, there is no shortage of people or job openings at any given time in any field. While it may not be those specific examples I gave in this instance, hopefully they help show how easy it is for a field in a city to have a high saturation of one group of individuals ((real life is probably a mix of all three examples and more)) (╹◡╹)