Immigrant acquitted of murder in Kate Steinle shooting is not competent to stand trial due to mental illness, evaluator say

A court evaluator concluded Jose Ines Garcia Zarate is not competent to stand trial on federal gun charges “because of a mental illness that is not presently being treated,” according to a court document from the US District Court in California.

Garcia Zarate, an undocumented immigrant, was acquitted of murder in the 2015 San Francisco Pier shooting of Kate Steinle, a 32-year-old medical device sales representative.

He is still facing federal criminal charges including possession of a firearm and one count of an immigrant illegally in the United States in possession of a firearm and has pleaded not guilty to both.
How can he be incompetent and plead not guilty ? Wouldn't he have to understand the criminal charge ?