The small group of people worth a dozen digits has hit a record high, just as Elon Musk approaches trillionaire territory. These 20 “centi-billionaires” have a collective net worth of $3.8 trillion, ... "100%" is equivalent to "all".

Understanding the Context

There is no rounding with "all"; either you get all of something or you don't. If a product advertised itself as "kills all bacteria" and then you found that there were 3 bacteria that it didn't kill, it doesn't matter whether that's 3 out of 10 or 3 out of 28 million; it's not all of them. Even in ordinary conversation, if your child says "I picked up all the ... “A 100” would be ‘a one hundred’… but more commonly (at least if you're talking about an increase of 10,000%, rather than a hundred individual increases of unspecified percentages) you would say increased (a) hundredfold instead.

Key Insights

As you note, "percent" means "for every hundred," so there is nothing at all wrong with percent values greater than 100 when discussing proportions, e.g. profits increased by 120%. Like fractions, however, percent values are longer to express than multiples, so for values much greater than 100 you may hear them less frequently: Yes, the correct usage is that 100% increase is the same as a two-fold increase. The reason is that when using percentages we are referring to the difference between the final amount and the initial amount as a fraction (or percent) of the original amount. Why is "a 100% increase" the same amount as "a two-fold increase"?

Final Thoughts

4 I would use 100% when it was an actual measurement, and one hundred percent when it's an expression. After counting, I saw that 100% of the visitors wore hats. At the time, I was one hundred percent sure of my observation. Later I found out that most were not actually hats, but pets. word choice - Choosing between "100%" and "cent percent" - English ...