That seems like a very broad claim that can't possibly be true without some sort of qualification. Are you saying that there are people who know 100% what they're doing 100% of the time in 100% of endeavors across every category relevant to living life? I feel like what you mean to say is that there are people who know exactly what there doing when it comes to X, where X is a well-defined, specific subject-area.
Assuming that statement is true with the scoped qualifier, even that wasn't always true for this hypothetical person. They had to have learned how to do X and gained enough experience over time to know exactly what they're doing in the matter. Which means, there was a time when they didn't know what they were doing. And I'd say this hypothetical person at that hypothetical time before they knew what they were doing is really the target audience for whom the article was written.
Early in life you get the impression that parents know everything, then it's teachers, then it's bosses. When in fact they don't.
We're not robots with perfect abilities, understanding that everyone to a degree doesn't know what they're doing gives you a different outlook on the world.
Note, that doesn't mean everybody is incompetent.
How can you possibly make this assertion? I have a direct counterexample; I know what I'm doing. There's a lot of stuff I don't know, but I would not describe my state of being as "not knowing what I'm doing".
>everyone to a degree doesn't know what they're doing
This is very different from "Nobody knows what they're doing". Just because no one has a perfectly clear idea of everything doesn't mean they don't know what they're doing.
EDIT: Actually "all the time" doesn't quite cover it, I'd say it's "most of the time".
Then so does the statement "Nobody knows what they're doing.". The author has no idea what people know or don't know.
... Er, no. There's only so much complex, useful stuff one can produce, before you have to conclude, "okay, that person understands something."