I'm guessing that's a mistake in the article, in the video he says the setup costs $100

I wrote the blog post a couple months back, and was considering the parts for the "computer" (Pi and adapter board alone) in that $20 estimate—the V3 version of the Pico Micro Mac is an additional $6.

But I'm assuming in that setup you have a VGA-capable monitor on a shelf somewhere; not everyone does, of course.

In the video I mention the all-in cost, accounting for the monitor, keyboard, mouse, power adapters, etc.

This isn't a practical setup by any means, but at least you wind up with a neat couple bits of hardware that could be repurposed once you get bored of it!