First of all Bulky Rona is a check to Ninetales no matter how you see it. Which means that if Volcarona comes in for free against Ninetales, it will set-up on it 63,7% of the time (17,8% is Roar's usage, and 18,5% is Toxic's usage, which means that at best their combined usage will be 36,3%). And this is only accounting for the most popular set, BulkyRona, as ChestoRest can set-up on every Ninetales that lack Roar, if it comes in for free.
So now that we established that Volcarona is a check, let's see the counter part...
First of all you ask me since when do we mention something as a counter, if it is a counter only x% of the time. And i answer to you that we do it if the circumstances in which the counter works are realistic. For example, BulkyRona is a counter to any offensive Timid Volcarona without Fire Blast, and ChestoRest Volcarona is a counter to any offensive Timid Volcarona without Fire Blast, and to Roar-less defensive variants. These are all realistic scenarios that can easily happen. When you begin the match for example, lure a fire attack from Ninetales, and check the damage output. If it is Flamethrower and not Modest, then congrats, your BulkyRona can set-up on it. Same goes for defensive Ninetales, where you only have to worry about Toxic if you have BulkyRona (even if it Roar's you out, nothing happens to you, you are just unable to set-up on it). So i don't see where is the problem in informing the readers that they can counter some Ninetales with their Volcarona...
In the c&c section of Terrakion it mentions Claydol as a counter, when CB Terrakion 2hkoes with X-Scissor. Why? To inform the user about certain counters that can work if some certain conditions (not unlikely conditions that happen only 2% of the time of 'course are met). So to me if Volcarona counters Ninetales 33% of the time, it is certainly a reason to be mentioned.
Finally many pokes are mentioned as counters to x poke, if said poke is lacking some moves. So i don't see any reason not to mention that BulkyRona is a counter to any non Fire Blast, non Toxic Ninetales (HP Rock is simply too rare to get a mention imo).