There are several reasons why old docs work. First, release velocity approximated documentation velocity. If you only released once a year, your docs had time to be polished. Second, simplicity. Think of the length of the man page for ls in Seventh Edition UNIX vs today. The constraints of the time helped here in that writers needed to get their point across in one or two 72x24 screens, not two million pixels.

Since good documentation creates a consistent mental model in the reader, cultural affinity of the writer to both source (developer) and reader helps, and the old, much smaller, computer industry was able to pull that off. I sat two cubes from my doc writer and we shared the same cultural worldview with each other and our market. It's much easier to communicate in that milieu because so much can be left unsaid.

Its possible that we are entering a Golden Age of Text, where everyone realizes that they have to feed their AI with decent information in order to have any hope of it producing good answers (especially true for complex technical products and internal corporate processes). But I am not hopeful.