You would be correct in saying this, however, with more prominent threats such as Lucario,Metagross, Flygon etc rising, there is more pressure on Diancie to the point where it becomes more of a burden to build around. Most of the Hazard removers in the tier can easily outstall Diamond Storms PP and freely remove hazards meaning its role as a rocker is slowly fading and being outclassed by the rising likes of Necrozma or Nidoqueen who do match up well into most removers. Diancie is still obviously good, it compresses the likes of Obstagoon and Incinaroar however outside that it is slowly falling.
I'm not on the VR team, but this feels inaccurate. Diancie is weak to threats like Lucario, Metagross, and Flygon, yes, however its competition in other specially defensive stealth rock setters such as Stakataka and especially Registeel also is. Sure its not a Steel-type but it definitely competes with them—If Diancie is run if oftentimes takes the place of these Steel-types. Registeel's bulk is a bit better overall, it resists Fairy and Psychic, making it an effective Gardevoir answer, its not water weak, meaning its less afraid of Water-types. Stakataka resists fairy and psychic too, and is more threatening so it can pretty reliably land Toxic on important bulky targets such as Milotic and Gastrodon.
Yeah it struggles to beat Defog users like bulky Crobat one v one, but Stealth Rock still has more PP, so in that regard it can keep hazards up. Asides Xatu it pretty reliably can get SR up. The thing that does actually suck is Togekiss easily beating Diancie 1v1 if it lacks Toxic or Power Gem>Diamond Storm. I guess it does compete with Specially Defensive Nidoqueen somewhat as a SR setter, but its much more comparable to Registeel and its just not very similar to Necrozma as a SR supporter.
While Diancie has fallen out of favor its arguably gotten better as of recently because of Obstagoon having become a top threat. With the more recent rise of Obstagoon there's probably a valid argument for it to be A- though it staying where it is seems very fair.