Not that surprising if you consider that books before the Gutenberg printing press were artisans work of art that required years of work of specialists.

In other words: Today some of those will cost more than a Ferrari to make. They use Vellum paper that is much better that today's but require killing hundreds of animals each.

Only very rich people could afford that. I had access to European books collections of the 16th that are in Color, much much better than any normal book we have.

If you think about that it is normal. Color require more printing plates in a printer, but just changing your ink if you do it manually.

Not at all surprising. There were some "unshakable truths" in typesetting that basically held up from Gutenberg until the Internet came along. Knuth (partly through his friend Zapf) is well aware of them and respected them in TeX.

It's relatively recent that we've found out some of these "universal" rules might not have been so important all along and together with technology as another factor things changed.

It's not unlikely that Donald Knuth looked at examples of 16th Century typesetting when he came to design TeX. Or looked at examples of typesetting that had been influenced by 16th Century typsetting.

Really? Because what TeX did was make it possible to write "proper" books or formal texts via a computer - that was the whole point

A 16th-century formal book like this would be the gold standard to replicate if you want to make "serious" texts. And yes, in scientific literature, the "serious" text is a narrow target and far narrower than you might expect from the possible variation in a handmade artisanal work. Mostly because when everything is "custom", standardization and regular structure is exceptional