I can't speak to Satterwhite, but Brassard could produce 5-6" of depth from anyone who was over 1" erect, without a skin graft. Erect size does not translate directly to depth. In addition, when having sex, the whole penis can't be inserted (bodies get in the way), so depth does not need to match the partner's size, either.
The average cis woman has about 3-4" depth when fully aroused and stretched, and most of them have no trouble with the majority of possible partners. (My wife is 4" - she got curious with my dilators - and I was 6", but I never noticed an issue at all.)