I don't think it's a misconstruel of the study to say that it condemns physical punishment.
The fact you found it necessary to physically discipline your kids and grandkids does nothing to change the study's findings that you thereby put them at greater risk of mental health disorders and anti-social behaviors while not significantly increasing the likelihood that they would comply with your orders.
My kids turned out quite fine, thank you, and so are our grandchildren. None of them are shrinking violets, all three of our "kids" went on to college, and two of our oldest grandkids are starting college this fall on academic scholarships. And none of our kids and grandkids have killed anyone since yesterday.
And, just to repeat, I've seen all too many parents who have use the "time-outs" and groundings on a regular basis, and all too often I've seen the kids overwhelm those frustrated parents. One thing that's worse, imo, than a little slap on the butt, is a child who is not at the age whereas one can reason with them, whereas there's a dragging out punishment, such as in a "time-out" or being "grounded".