A small group of people that questioned the 9/11 attacks referred to themselves as the Truth Movement. Some people referred to this small group as “truthers”. It became a lazy and dishonest slang among a very small number of people to refer to anyone who questioned any of the official explanations of 9/11 as “thuthers”. That is what you are referring to. Normal people do not use the word. It is used dishonestly by a very small number of people.
The word was used dishonestly. It had no meaning. It meant whatever the user wanted it to mean.
Now this author makes up the word trutherism which he says he “really hates” to describe something that has nothing to do with 9/11.
It is a made up word. It means whatever the author wants it to mean. It is not in a standard dictionary. It has no common usage or commonly accepted meaning.
I’m sorry, but you’re wrong. The word “truther” has been in common usage for years now, used by a great many people to describe the various “instant conspiracy” movements that keep popping up. It is not limited to a small, politically motivated group. When I use the word in conversation, I’ve never gotten a blank look, because it is pretty universally understood. That alone makes it a legitimate word, and it is not going away, even if you don’t like people using it.