For basic residential plumbing work the moat is not knowledge. There are already books and YouTube videos that will teach you everything you need to know. Professional plumbers can't stop that. The real moat is that most people don't have time, don't want to buy tools, and don't want to get shit on their hands.

For new construction and commercial work the moat is a contractor's license. They don't allow LLMs to take the licensing exam yet.

How trades gate keep is time. You can’t just become a plumber or electrician on your own. You have to be an apprentice for years no matter your knowledge or skill. This is how it works in trades and unions and where the term “pay your dues” comes from. Like you have to literally pay($$) dues for years before you can move up.

There is a difference between something being a hidden/gate kept trade secret and something being easier for person X to do than person Y through a combination of real world experience and practice.