The NCSE is an organization to educate the public & the press about evolution & climate change. They wrote a satirical article, “Gravity: It’s Only a Theory,” to demonstrate what might happen if a science denialist refuses the existence of gravity using the same “logic” that they use to reject the existence of evolution.
NCSE does not deny gravity & it is mentioned at the beginning of the article that it is a satire. Flat-Earthers failed to notice that, and it became a hoax that they use to support their false belief. The author probably did not expect there are far more extreme denialists that can go as far as to reject gravity & even beyond.