In 1979, the United States made a commitment in the China-U.S. Joint Communiqué --
"The United States of America recognizes the Government of the
People's Republic of China as the sole legal Government of China.
Within this context, the people of the United States will maintain
cultural, commercial, and other unofficial relations with the people
China therefore claim that US must refrain from having official exchanges with Taiwan. Pelosi is the leader of Congress, protected by the governmental flights, so her visit is "official" by default.
How did Pelosi and US justify her visit to Taiwan considering the promises made in the 1979? Is the visit an official or unofficial one?
From the quote alone I don't think US promised that there will be no official interaction between Taiwan and China.