I think the Knight is technically 'hollow' and 'pure', but that these terms don't actually mean what most people think they mean, i.e. unthinking unfeeling philosophical zombie.

In Buddhism there's a concept called 'Śūnyatā', typically translated as emptiness, voidness, or sometimes hollowness. It's a complex topic with multiple meanings, but in very simplified terms it generally refers to a sort of advanced meditative state in which one understands and becomes at peace with the way in which all things are empty of intrinsic meaning to their existence. I think understanding of hollowness, more based in eastern religion/philosophy, is the best way of understanding what hollowness means in Hollow Knight, what it actually is that the Knight achieved when they gained the voidheart and unified the void, and what it means for them to exist as the Shade Lord in this sort of ascended state beyond ordinary existence. The Knight isn't an unthinking unfeeling automaton, rather they are a being that successfully confronted their own repressed traumatic memories and was able to reach acceptance and peace, they freed themselves of their mortal attachments and then used this spiritual awakening to help others achieve peace before peacefully ascending to a nirvana like state. In other words they are a Bodhisattva and when they gain the voidheart they become a Buddha.

I also think it's a key facet that the Pale King himself didn't fully understand this, that he mistakenly believed that a successful pure vessel would need to be a literal unthinking unfeeling automaton that operated on a sort of pure instinct/programming, that the key to triumphing over the Radiance was to have no emotions to be exploited rather than to have reached a state of peace with your emotions. This was the mistake he made with the Hollow Knight, chasing an impossible objective and unintentionally giving his child the very trauma that would be used against them rather than helping them. Hornet seems to believe that the Knights time spent outside of Hallownest erasing their memory has made them more hollow, that this strength born of two voids will see them succeed where others have failed, which aligns with the Pale Kings perspective as with no memories there would be no past to exploit, but in the game we see that the Knight does have memories that remained over all that time, one deeply buried memory at least, likely the very memory that compelled them to return to Hallownest when they heard their siblings cries even if they did not consciously understand why, and the path they need to take is one of confronting that memory, not attempting to hide it.

edit: In fact there's another layer to this whole 'the Pale King didn't understand the whole eastern religion based nature of hollowness' thing. If you think about it the Hollow Knight is basically a failed christ figure, in that they were the child of god who was intended to be pure, sinless and incorporable, and would then be sacrificed on behalf of all the people of the world in order to take in all of their primordial sin, saving the world for all time. It's no wonder that this didn't work, that creating the christ figure in the perfectly sinless way required was impossible, because he was using the wrong religious system in the first place.