I think we underestimate them, but they also underestimate themselves (I hear a lot of "I can't..." by people who definitely can, once that word is in their head they no longer can but because of the mental block).
Also, there was a major difference between what the secretaries would have been using before troff and their experience with troff. In particular, at that time, they were probably using typewriters. A digital workflow would have been a massive improvement in experience. Need multiple copies? No more carbon copies or retyping, now it just comes off the printer (which may still use a carbon copy, but at least you don't have to retype it). Need to adjust something? At least for future printouts it's just changing the file, and maybe using whiteout and handwriting the correction in a few printouts. Want to reflow the entire page? On a typewriter you start over, with troff you edit.
Today, people have Word and Google Docs, why do they need troff/Markdown/TeX/LaTeX/Asciidoc/org-mode/etc.?
The experience, for the user, is not as qualitatively different as those secretaries would have experienced at the time. Not just secretaries, my parents (a pilot and an engineer) took to computers in the 1970s (in college)/1980s (work, later home) for the same reason, it was a better experience than using typewriters and hand calculating things (even with a calculator).