My point is simple. IMO if there were other people in her niche that were using SEO, both on-page and off-page, then her site would NOT be the market leader.
I think she is probably in a non-competitive niche with very little "real" competition.
What should also be taken into account is that off-page SEO is designed to mimic what occurs naturally i.e. building backlinks deliberately in a way that would normally happen naturally in an ideal world.
I don't know of any site that had a good ranking in a competitive niche worth decent money prior to this update that did not engage in any type of SEO work EXCEPT where there was no "real" competition.
By real competition I mean webmasters who understand how to use SEO.