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A pencil point is a tapering edge, so I'm assuming the author is considering <1mm for "fully capable computer the size of a pencil point" being unreasonable.

Microcontrollers are not a huge amount larger than that any more:

-Cortex-M4f in 1.6x1.6x0.65mm (MAX32660)

-Attiny20UUR in 1.56x1.4mmx0.50mm

Bare dies may be a bit smaller than the above.

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I would note that thinning and encapsulation of bare die allows typical thicknesses of <200um. Now, if you need an smt package and a (F)PCB then everything blows up to mm total thickness. Note also that typical minimum dicing width is 6-700um and your micros layout could be re-done to be a more toothpick 3x0.66x0.2 size. Still, gotta get power in and signals out, but conformal parylene will cover ACF or wirebonds.

You can put a lot of tsvs across the border of IC and grind up to them, hoping that the wire will bond to at least one of them. And if you do bonding by hand, you can bond right to the metal if the oxide below is thick enough.

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