Just curious do they ever post before/after photos of the hollows actually being filled in?
Those hollows seem to be present at birth for some horses. I don’t doubt some hollows are cause by poor saddle fit but what about the horses who don’t have riding careers? My breed of choice (TBs) have them - even in young/unstarted stock. TBs don’t own the cornerstone of hollow wither pockets - I see WBs with them too. It begs the question of what they think of stallions like Gun Runner or Into Mischief - top racing stallions who do not have a riding career but have the hollows.
N=1, this is my really radical take on the hollows. I think that they, along with the somewhat abnormal musculature you see just above the trapezius, can be present in horses with cervical malformation. That does not mean the horse can’t be ridden - ECVM seems to be in just about every breed - but I wonder sometimes if we are seeing the physical impact of abnormal muscle/ligament attachment because of abnormal or absent processes.