Answering my own question (I think):
I have probed a bit more inside of the OEPocketsphinxController+RapidEars.h and saw that RapidEars is using Objective-C `category’ feature to extend the class. This was a great opportunity for me to learn about the ‘category’ feature of Objective-C.
As I researched more, I found this on Stack Overflow:
And doc from Apple:
ANSWER: Yes, the -ObjC linker flag is indeed required when using any plugins for OpenEars. :-D