That doesn't surprise me at all. Isn't software engineering in essence about being constantly confronted with new problems to solve and having to come up with a sufficient one on the fly? It seems very hard to estimate this, even if you know yourself well.
They were 20% underestimating how long it took them to do a 1-8 hr task that they had just completed.
It's like Tog's study that people think Keyboard is faster than the mouse even when they are faster with the mouse. Because they are measuring how they feel, not what is actually happening.
https://www.asktog.com/TOI/toi06KeyboardVMouse1.html
That is a very weird set of findings.
This one in particular:
> It takes two seconds to decide upon which special-function key to press.
seems to indicate the study was done on people with no familiarity at all with the software they were testing.
Either way, I don't think there is any evidence out there supporting that either of keyboard-only or mouse-only is faster or equivalent to keyboard+mouse for well known GUIs.