It's really fun to see that the people that we consider geniuses are really just smart people spending their lives forming concepts into reality, one function at a time, like the rest of us.
OTOH, smart people often build systems with excessive complexity, leading to maintenance problems. Genius is found in making complex things comprehensible and simple.
I would say that people who build systems with excessive complexity are (almost per definition) not smart. They made appear confident and have a high level of "I am so smart" feel about them. But they are clearly not.
Some people learn the value of simplicity through experience. They start out building over-complex things, and they’re not “not smart,” just inexperienced and misguided.