While it's true that some non-native speakers have better language awareness than some native speakers due to the fact they have explicitly learned the language, it is in my opinion rarely the reason that they get hired in china as an ft. The usual reason is that applicants from non-big 5 countries can be hired on the cheap. As for Filipinos in particular, I have heard Chinese say that they like to hire them because as fellow Asians they can adapt to the culture more easily. I think you can read in between the lines on this one as what they really mean (as you already stated) is that they understand their place and as a result are easier to control.