We are looking for a possible reason why the author included Anthony Johnson and his family in the passage.
The author mentions Mr. Johnson in lines (49-63) so we should at least read these lines unless we are confident with our answer choice. However, it is also good to briefly review with the lines preceding 49 to know the context of the example. In lines 40-44 the author mentions that the authors underemphasize that customary law was closing in on free African-Americans before the 1670's. This is crucial to the interpretation given from Innes and Breen about the reason why Johnson left. He/she mentions that this may have not been the only reason, or possibly even the real reason. The author then goes on to say that the institution of slavery was potentially harmful to his descendants in Virginia.
So, though Innes and Breen dismiss this, the author provides this examples as a possible shortcoming to their interpretation.