"Even if one allows that the lower edge of the ribcage is part of the chest, it is certainly nowhere near the nipple - unless we're talking about a really old couple."
Most couples include a female partner. Females tend to have some glandular tissue on their chest wall which protrudes to one degree or another, with a wide variation from individual to individual... In many cases that means that if the "skeletal" contact (muscle-padded we hope) is at the lower edge of the ribcage, there will be "soft tissue" contact above as well.
"whatever Jonathan may say (I can't find the notes which you mention), what he and Melissa do is quite clear to see."
Indeed, they keep their shoulders quite parallel, which limits the amount of change that can occur lower in the torsos. Their ability to maintain this parallelism was better demonstrated in a promenade specific example from a few years back (I think it was american foxtrot) not prominently features on this website than in the whisk video, which does look a bit turned out to my eye. It may just be that the whisk, by virtue of having the opening-prone CBMP position occur before a clean promenade configuration can be established rather than after it already exists, is just more prone to this than the more flowing promenades of the other cases.