It’s like saying the Eiffel Tower is just a tower.
His article links to this site which explains what scientists mean when they use the word theory.
In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It’s a well-substantiated, well-supported, well-documented explanation for our observations. It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It’s as close to proven as anything in science can be.
So by saying “evolution is just a theory” what you are saying scientifically speaking is that evolution is as close to being proved as it can be. Which is what mainstream scientist have been confirming for the last 150 years.