It totally is. Someone migrating, say, from Changsha to Guangzhou is going to face a significant language barrier interacting with locals. Yes, they probably both speak passable Mandarin, but possibly with quite different accents affected by Xiang and Cantonese, respectively. An influx of easily recognizable outsiders is going to fuel resentment wherever they compete with locals for scarce resources like jobs and living space. The Chinese household registration system 户口 hùkǒu exists in part to keep that internal migration under control.
Internal migration poses difficulties, but it is not the same thing as immigration. People immigrating to China would probably not speak a language that is mutually comprehensible at all. They would have a much more different culture than internal migrants. But most importantly: immigrants have no claim to being stakeholders in China and how it is governed, whereas each and every citizen does.