This was all experts driven in that time to my knowledge.
Empirical validiton did not really take off until the late 00s.
https://hci.stanford.edu/publications/bds/4p-guidelines.html
Don had the explicit expert knowledge first stance in 2006 and 2011, nothing inherently wrong with that, but it's defenitly no research driven.
"Always be researching. Always be acting."
https://jnd.org/act-first-do-the-research-later/
Tognazzini and Norman already criticized Appple about this a decade ago, while the have many good points, I cannot shake the feeling that they simply feel like the were used to just brand Apple as user friendly in the 90s and that Apple never actually adopted their principles and just used it as it fit the company's marketing.
https://www.fastcompany.com/3053406/how-apple-is-giving-desi...
there are a bunch of discussions on this
https://news.ycombinator.com/item?id=10559387 [2015] https://news.ycombinator.com/item?id=19887519 [2019]