It's a symbiotic relationship but it's as much if not more about Rousey and her team than about the UFC marketing machine.
UFC provides the platform, but Rousey and her team have driven her to levels of fame far beyond what has historically been possible.
We shouldn't forget that Rousey was already on the ESPN body issue and Maxim BEFORE she signed with the UFC. Rousey already had a Showtime special about her and was on ABC news BEFORE the UFC created the 135 division.
If it was all about the UFC hype machine, we would have seen a LOT more cross platform stars before Rousey, or are we assuming that the UFC just never tried to promote their previous stars?
It would be a lie to say that Rousey's success hasn't been partially driven by the size of the platform and audience that the UFC provides, but there have been dozens of UFC champions. It was Rousey's particular set of attributes and in ring dominance combined with the promotional savvy of Rousey and her team that have taken her to her current level of popularity.
It was Rousey and her team that seek out promotional opportunities outside of the ring aggressively. It was Rousey and her team that sought parts in movies, signed a book deal, sought out magazine covers, mainstream media interviews, etc, etc.
It was Rousey and her team that sought out William Morris Endeavor instead of the usual shoe box managers and agents that most fighters retain.
Yes, Rousey isn't Rousey without the UFC. However, even with the UFC there is clearly only one Rousey.