There is no mechanical difference between the Legends of the Shining Jewel campaign setting rules and normal Pathfinder rules, just some flavor differences for racial traits, some different races, and some variants for classes. It mostly is just a setting for a living campaign run by a small group of friends that have been building this world for a very long time. Because they are such a small group, that does mean that it is highly likely that the module writer and the GM running it for you are the same person so there is plenty of opportunity to get creative in how you solve problems, which is a big plus compared to modules from say Pathfinder Society where things tend to be a bit more railroad-ey at no real fault to the DM.
I have only ever played in the campaign at Gen Con and so have seen that communication is not one of their strong suits outside of their more immediate group, but at least in years past they were good about including the level restrictions (if any) in their descriptions. I suspect that someone else has taken over for generating their submissions as they did have someone in their group pass at the end of 2018, and including the level restrictions in the submission description slipped the new person's mind. Hopefully you have not become fully dissuaded from participating as they really are a bunch of overall nice and highly entertaining people with some very quirky and fun modules.