Also might be worth noting that I went back to check in Castellan, and the Masque's feat would actually be lower-end 4-B as opposed to higher-end High 4-C, as the supernova caused was explicitly large enough to consume the whole system while moving at nearly the speed of light. There was also a warp shockwave which engulfed everything even before that.
"Crowe bore witness to Angriff going supernova. At first, the star flickered, the moment of its death a phenomenon utterly removed from the materium. The flicker was the stain of the empyrean. After the flicker came the blaze. Angriff shone more brightly than it ever had before. It seared the void with the brilliance of its pain.
The star exploded. Its outer layers expanded across the system, a sphere of absolute annihilation, swallowing the inner planets, erasing all trace of their existence, and the hundreds of billions of souls who had lived upon them. Civilisations that had endured for thirty thousand years only to be destroyed by the dance of the Masque now vanished even from memory.
A monster worse than the all-consuming fire raced ahead of the stellar material. It was a warp shockwave. It distorted space. It made the void bleed. The colours of the immaterium, screaming corruption and madness engulfed the system, one final cry of power from the daemonic engine. It arrived to consume every psychic life force in the system.
The warp wave overtook Angriff Primus. Crowe saw the first moments of the planet's final damnation, its transformation into a burning nightmare." - Castellan
Furthermore, looking back at Eye of Terror, the Rose Cluster is stated to be a relatively unremarkable globular cluster.
"The Rose Cluster was, as its name implied, a large cluster of stars. Typically these were globular and contained thousands, sometimes tens of thousands of stars. In that respect, the Cluster was unremarkable." - Eye of Terror
Globular clusters do not contain as few as "thousands" of stars (author was likely thinking of open clusters with the lower end). They are packing 10,000 at a minimum and can have up to several millio. The lowest result for the Rose Cluster would actually be using 10,000 stars, and even if it was a small globular cluster (which the story suggests it isn't), it would far more likely be packing 20,000+. Due to his confusion, I wouldn't say the author intended for the Rose Cluster to contain millions of stars, but if it was intended to be a globular cluster, 10,000 is the bare minimum, and tens of thousands is consistent with its description.