This has also been my experience writing documentation.
You write "hold button for 5 seconds until light is blue", and someone will hold the button for 3 seconds until it turns white and ask why it's not working.
You don't know the difference between white and blue for sure until you've seen both. I actually recently had more or less this problem trying to set up my cable modem (maybe it was something like is it blinking, pulsing, or flickering, but anyway).
I try to set an example by providing copious context but I don't feel like others imitate me. Sometimes I think I'm the only one who has a theory of mind.
I'm color blind so don't blame me. If it turns white then blue I can probably see that, but if it turns white and there is no warning it goes through white before blue I'll assume you just got a different led on the line.
It's fair to assume that the docs are simply wrong/outdated about the specific duration and color when it works, but I would start to question those assumptions when it doesn't.
Pedantically proving instructions wrong (e.g. with a stopwatch and colorimeter) is a great way to submit a useful bug report with solid steps to reproduce, and a lot of the time it ends up solving the problem as well.
20 seconds is long enough that I will not wait that long in general. When something else happens in 3 seconds that seems like it could be right I might wait 5 more seconds when it doesn't work, but not 20.
Did it need all 20 seconds? If so, the docs were definitely wrong and useless and I see your point.
My suspicion is that it works fine with 5 actual clock seconds (instead of "a short while"), and that they just said 20 so that even the most chronologically challenged would end up holding it for at least 5.
You write "hold button for 5 seconds until light is blue", and someone will hold the button for 3 seconds until it turns white and ask why it's not working.