Kind of true - what's lacking in most of use is intent, discipline and commitment towards personal projects. First, you need to start a personal project. Then, you need to make sure you complete it. Unfortunately, all that isn't as easy as it sounds.

There are levels of intelligence inbetween the general population everywhere. Some are highly intelligent, in ways not easily explainable to the lesser minds.

The advantage of these people over the rest maybe diverse, some edge on maths, other on memory, others on empathy, others maybe in music, sports.

But the advantage is decisive, tangible in actions and facts (in retrospective, obvious), even if not all of them reach "authority" levels, the sheer ideas most introduce in an otherwise anachronic current status quo, are enough to make the "authority" to make a change of course.

Once a new idea was analyzed by lesser minds it's for them reasonable to implement it not minding a lot about where the ideas came from, nor why or who make them come to the attention of the "authority", so they just "complete" the action pattern set up from an external idea.

In the great scheme of things, many core decisitions probably relayed on the thinking of very smart people which created the path followed by others.

Think Da Vinci, Einstein, Von Neumann.

There were many (there are), many of them, most completely unknown to the history.

Betteridge's Law of Headlines - no.

Unless you are the smartest.

You present a conclusion ( can people of average intelligence build good things) and the premise leading to the conclusion. (Existing systems were built by people of average intelligence)

The conclusion is true. Average intelligence is sufficient to build good things. The premise is false. People of greater than average intelligence contribute greatly to projects started and operated by people of average intelligence.

I'm not sure how to respond because I don't know what you mean by "systems." Can you offer any examples of systems that were built by average people?