I used to believe such news. But after watching a recent documentary on Youtube about South American illegal immigration and suddenly a Chinese guy appeared among them, trekking from Colombia to Guatemala, going through a dangerous mountain alone with other South American, and the documentary team could easily interview him in Mandarin and he started badmouthing China (In Mandarin too), Everyone with a sound mind would easily understand that the Chinese guy did not exist and all were just planned and arranged by the US documentary team in advance.
So can be said about similar news on Chinese immigrants. Probably those are the results of the last year? $500 millions approved to spend for anti-China news by the US Congress.
Similar news will appear again with higher frequency in the future. And with 500 millions, they can employ thousands of Chinese/Vietnamese American to play/act as Chinese immigrants.
And I do not say there is absolutely no Chinese who still want to immigrate illegally to the US.