It's obvious that it does not make sense for a plumber from the UK to look for work as a
plumber in China. Why would a plumber then go to China, eventually holdong a a fake
degree cert, and then eventually work illegally?

What kind of parallel is that?

What you have said here may be blindingly obvious, but how can you compare that to ESL teaching in China, when most ESL teachers were not teachers in their home country?