Schopenhauer affirmed that his philosophy was known even before Kant, in the Upanishads:
The view of things […] that all plurality is only apparent, that in the endless series of individuals, passing simultaneously and successively into and out of life, generation after generation, age after age, there is but one and the same entity really existing, which is present and identical in all alike;—this theory, I say, was of course known long before Kant; indeed, it may be carried back to the remotest antiquity. It is the alpha and omega of the oldest book in the world, the sacred Vedas, whose dogmatic part, or rather esoteric teaching, is found in the Upanishads. There, in almost every page this profound doctrine lies enshrined; with tireless repetition, in countless adaptations, by many varied parables and similes it is expounded and inculcated.
— Chapter 4, of “On the Basis of Morality”
I would say the reason that Schopenhauer considered the noumenal as an irrational will that has primacy over the intellect was because he did not study Samskritham himself, and thus thought that is what was mentioned in the Veda-s. In actuality, the Veda-s say that the ultimate will is beyond rationality or irrationality, but it is the ego that is irrational. The will of the ego is to be transcended, but the ultimate self is beyond will, and it’s will is not to be transcended.