I think those theories are missing the main point. Going to the gym is not fun, because gyms have a crappy user experience. So let's say you join a gym, but don't know very much about working out. You walk into a room with hundreds of machines, and are expected to know how to structure a work-out, how each machine works, be familiar with all the muscle groups and exercises, and also really care about planning out and…
As the article points out, the gym wants you to come once (not have fun) and then not come back - but continue to pay the monthly membership fee.
If the gym was fun, and every member came in regularly, the gym would go out of business. Their goal is to make the gym seem like something you should have a membership for (lots of high-tech looking machines and posters about health and attractive staff), but actually not fun enough for you to come more than a few times a month.
This is why there is a rise in the small group-focused workout clubs (that emphasize fun).