Here are some reasons. I'm not familiar enough with aircraft to argue either way though
"Is China’s ARJ21 Just A MD-80 Copy Or Is It The Beginning Of China’s Rise In Commercial Aviation?"
"The ARJ21 is partially built on specs from the old MD80, thanks to the presence of U.S. manufacturers in China. McDonnell Douglas was operating an MD80 manufacturing facility in Shaghai prior to its merger with Boeing, thanks to a lucrative deal inked in the early 90s. During this time period, presses and other parts were shipped from the United States to the Douglas facility. This gave the Chinese access to Western technology.
Once Boeing merged with McDonnell Douglas, production of the MD80 ceased, marking the end of an era. After abandoning the final MD80 and MD90 assembly lines in China after about 30 frames, Aviation Industrial Company inherited McDonnell Douglas tooling."
"It did not take long before China was announcing it’s “new” regional airliner project, unveiling a design that was eerily similar to the MD80 design. COMAC officials in Shanghai vehemently defend the ARJ21 as an original design. However, despite COMAC’s protests to the contrary, it is commonly accepted that the ARJ21 is fundamentally a redesign of the DC9. "
https://www.avgeekery.com/is-chinas-arj21-just-a-md-80-copy-...
"Is China’s ARJ21 Just A MD-80 Copy Or Is It The Beginning Of China’s Rise In Commercial Aviation?"
"The ARJ21 is partially built on specs from the old MD80, thanks to the presence of U.S. manufacturers in China. McDonnell Douglas was operating an MD80 manufacturing facility in Shaghai prior to its merger with Boeing, thanks to a lucrative deal inked in the early 90s. During this time period, presses and other parts were shipped from the United States to the Douglas facility. This gave the Chinese access to Western technology.
Once Boeing merged with McDonnell Douglas, production of the MD80 ceased, marking the end of an era. After abandoning the final MD80 and MD90 assembly lines in China after about 30 frames, Aviation Industrial Company inherited McDonnell Douglas tooling."
"It did not take long before China was announcing it’s “new” regional airliner project, unveiling a design that was eerily similar to the MD80 design. COMAC officials in Shanghai vehemently defend the ARJ21 as an original design. However, despite COMAC’s protests to the contrary, it is commonly accepted that the ARJ21 is fundamentally a redesign of the DC9. "
https://www.avgeekery.com/is-chinas-arj21-just-a-md-80-copy-...