To be fair, that's most likely old continuity and the fact that it is stated that Shiki Tohno's eyes will outgrow that and he will die eventually due to info overload in comparison.
Though don't mistake that for non-canon(Type-Lumina explicitly goes with it still being canon overall) but Shiki Tohno does do some shit in the Remake with MEoDP that's on Ryougi's tier and it's basically MEoDP rather than Pure Eyes that ultimately became such due to his brush with death.
Even in Lumina, Ushi goes out of her way to call them “Pure Eyes”.
(also tbh, that death boundary is far better than anything ryougi has done with her eyes)
There’s also a paragraph from KT that implied that Shiki was the result of the Nanaya’s research being successful,rather than something purely innate like Ryougi
「究極」= ultimate, finality, or absolute end
Though their powers are similar yet fundamentally distinct, it may be misguided to compare them. Even so, Kiri understands he falls short by that very standard.
Only those who have made "death" a finality can oppose this demon.
But Kiri—who merely researched death and lacked it from the very beginning—could never ascend to such heights.
If anything could rival this demon, it would be something broken from birth, just like the demon himself.
It makes sense for pure eyes to be related.