That wasn't the point. The point was that you don't go from walking the baby to sleep to putting the baby down horizontally. You sit down for 5-8 minutes first, during which time the baby transitions into a deeper sleep that won't be disturbed by being laid down.
If you go from walking to putting the baby down horizontally, the baby will often be awakened by the sensation of falling backwards, and wake up.
I do. No one is saying "no parent has ever tried this genius idea". They're just saying that it's useful to have a study on it, and parameters around how long one should remain sitting before attempting to lay the baby down. Also, you're moving the goalposts. Your original comment said derided the idea about walking a baby to sleep. But that shows you missed the point of the study, which was not about walking at all, but rather the next step.
That step is apparently not super obvious because this grabbed over 300 upvotes on HN, no small feat.
Even if it was obvious to some, parents of newborns are often (always?) sleep-deprived, so it might be hard for them to remember how well it worked to hold the baby still for 2, 4, or 6 minutes. I didn't have especially difficult sleepers, but it would have been nice to have seen this data back when we had tiny ones.
If you go from walking to putting the baby down horizontally, the baby will often be awakened by the sensation of falling backwards, and wake up.