The word “theory” has multiple meanings and can be ambiguous. Its meaning in a scientific context can be different from that in the context of everyday communication. Flat-Earthers erroneously assume we call it a “scientific theory” because it is still unproven. In reality, in the scientific context, a theory is commonly regarded as correct, & we call an unproven supposition a “hypothesis.”
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.
The fallacy of ‘appeal to definition’ is using the definition of a term to support an argument as if the term cannot have other meanings or even conflicting meanings. Flat-Earthers often use this fallacy, for example, over the word “theory.”
A communication problem can occur when a term gets misinterpreted to mean other than what was intended. A simple clarification should quickly correct the problem. The appeal to definition arises if the clarification is refused, and the person insists on using the wrong & unintended meaning of the term, and use it to support their arguments.