I read the primary study and admittedly skimmed the very long article. So far as I can tell the takeaway is: after baby is 6 months old there are no observed downsides to sleep training, is that correct?
To be clear, on my read there were no difference as far as downsides, it seemed to improve the mental health of the parents and it taught the babies to return to sleep without needing intervention from the parents, which I would argue are major upsides.