This isn't new though. They've always been trying to say stuff like Manchus weren't Chinese. Manchuria is a Japanese term after all.
The best way is to call them out and ask them to cite the terms in Chinese. Watch them squirm since they don't know Chinese. The original HQ of the Manchus was in Liaoning which last time I checked was part of the Yan state back in the warring states. So somehow the main part of Manchuria is owned by China. Also Nurhaci somehow was raised and taught by a Chinese general too. Why were they under Chinese rule again in the first place?
Also another trend I noticed is despite people insistence that Manchus and Mongols were not ethnically Han, people are realizing many rulers of countries in general like UK, Russia, Germany, France etc are not native either. Japan so called god emperor has Korean blood so I guess Korea has claim to Japan now. Yet no one doubts the legitimacy of them. They wanna try doing this with China, they will open a can of worms they suddenly don't want to apply to their own countries.
Recently I found out George Michael was never native British but is actually Greek. Oasis the most popular "Britpop" band are actually Irish when it comes to their original members and most notably the brothers. There's a lot more where that came from.