I don't find the comparison useful for a few reasons. For one, 'cult' can be one of those words that shuts down discussion, like comparing somone or something to Hitler and Nazi Germany. All possiblity of a rational, respectful discussion flies out the window as emotional buttons are pushed and the extremes are emphasized.
For another, abusive behavior is abusive behavior no matter the structure it occurs in. The behaviors RP describes are abusive and not to be tolerated. But put those same people in a non-hierarchal structure, and guess what - still abusive behavior. We've seen people be treated horribly in non-primary/secondary structures. If someone, or a couple, is capable of treating someone with respect, care and love, then they will do so regardless of structure. Yes, I can see that some unscruplous master or mistress would use the same techniques to manipulate and control that a cult leader would. That doesn't make the master/mistress' behavior cultish - just abusive dominants.
Many people make primary/secondary relationship work well for all involved. We don't hear from many of them because, probably, they are happy. Many people make non-hierarchical models work well for all. I fear that emotional eliding of cults are abusive, primary/secondary relationship can have abusive behaviors similar to cults, primary/secondary relationships are cults, primary/secondary relationships are abusive. I realize this is not where RP was going but it is an easy place to end up if one follows the argument.
Finally, like polyamory, cult is a word that gets used a lot in different ways. Hoever, using it broadly has the effect of hiding or de-emphasizing the dangers of actual cults and the damage they cause to people.