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For instance TSMC is discussed a lot on HN and every time I'm thinking that even TSMC itself probably couldn't produce their latest chips if they had to start from scratch tomorrow.

Yes, I think people create more internal documentation then they read.

It can be documents and it can be people, but it's not essentially either one. It can take many forms, including being lost when none of those forms has it on offer, as every business is different. An institution with excellent documentation, mature processes, and adept hiring could retain its "memory" without a single human member remaining from the past. Oral history and other humanistic forms of memory make everyone feel warm and fuzzy, but they're not to be idealized as the only real memory simply because they were underappreciated for a some time.