Also this is a management illusion. There is no evidence for this assumption. You don't even know the probability distribution. There is no reason to assume that the percentage is equally distributed across all firms or countries. And anyway, the article is about something else.
No, it's an axiom. It defines a way to make collective/collaborative entities hopefully bigger than the sum of their parts. I think of these things (corporations, groups, movements) as aggregate people, and that's very much what Google is about.
Google deals almost entirely with aggregate people: statistics, algorithms, collective behaviors, machine learning, implementation that's never about individuals but is about larger population trends. Aggregates, not special unique snowflakes.
As such this is not an illusion but an axiom. Google and entities like it (themselves humongous aggregate 'people') MAKE individuals replaceable, the better to be dealing with other entities like themselves. This is only going to accelerate the more they get to bring AI and machine learning into the mix… which by now is long established, nowhere more than at Google.
Maybe an axiom as in being something we assume (because we can't/won't figure out whether it is actually true) this and base our decisions on this axiom.
Those exceptional individuals are incredibly rare, like one or two in a generation. So you need to be Shannon-likes to be not replaced by some middle manager in a big corporate? Emm, if someone were this accomplished, why would they care about one employment? That is the wrong question to ask.
Truth is, if Google thought they were not replaceable, it would not fire them this easily.
Much more people than you expect have at least one exceptional skill; from my observations I would say at least 20 to 30%; the more extraordinary skills per person, the rarer of course. And if indeed the human workforce were really such a generic, easily replaceable commodity, why do most companies, including Google, go to such great lengths in recruiting, with assessment centers and so on? And why are there so many unemployed IT specialists, for example in Germany, when at the same time the industry associations claim that jobs cannot be filled?