Let's take a moment to parse this sentence carefully. It's true in two ways.
An employee that voluntarily shares his knowledge with his coworkers, keeps nothing solely in his head, but rather strives to document publicly the knowledge and know-how he acquires on the job, is the most precious employee the company has. He knows that if he quits tomorrow, he is totally replaceable, in the sense that there's nothing he would need to teach his replacement - it's all there in the open. No knowledge is permanently lost. This is the guy you never actually want to replace.
An employee that hoards knowledge and positions himself as the omniscient guru, to whom all must come for answers, should be the first one fired. Sometimes people do this unintentionally. They don't see a problem with having all the answers. That's why managers need to instill into them the following mantra: the stuff you keep in your head should only be a copy.
Let's call it brain cache.
What you bring to the table as an employee is your attitude, your ability to reason, make smart decisions and learn. Everything else is basically brain cache. A cache is a powerful thing, mind you. But if the problem of replacing an employee boils down to warming up a new brain cache - that's at least manageable.
I'm trying a new format in this post: highlighting sentences for readers to quickly skim the text. The idea is that if you read only the highlighted text you should get a succinct version of the post, but still in sentences that make sense together and form a continuous and coherent version of the text on their own.
As an avid blog follower, I find myself practically always skimming posts - sometimes hundreds each day. I don't really read any blog post word-for-word unless I know for sure it's interesting enough. Especially wordy ones (like this one). There are just too many of them. I suspect I'm not the only one who does this. If you're reading this (and you're able to see the subtle HTML formatting I've applied), I'd be interested to hear you opinion. Is this actually helping you read, or is it just annoying?