Did they knot have any subjects that were scouts or sailors?
They use a reef knot in the example image (which is also the thumbnail for me), which is pretty widely known to be strong as hell.
Edit: double checked, and they don’t mention having participants that were experienced with knots, or excluding them to measure only people’s intuitive grasp. It isn’t a big deal, but it would make a difference in the outcome.
Edit 2: The work itself has the info which includes identifying knots, which means non-naive participants still didn’t always guess the strongest knot. So, that would mean even familiarity with knots in general doesn’t lead to knowledge of their relative strengths or a more intuitive grasp of those strengths. In other words, most people, even if they can tell what a knot is by looking, won’t be able to guess the strength levels, they’d also have to already know that.
I think, in general, there are a lot less people who have hands-on experience with knots than there were a few generations ago. You mentioned scouts and sailors, but those are a relatively small portion of the population. Rock climbers would be another group with some specialized knowledge. But there are very few every day uses for knots aside from your shoe laces.