My understanding, prior to briefly reading that paper, was that Schopenhauer came up with the core of his philosophy before he came across the upanishads. When he did read them, he was delighted to find that they contained many of the same ideas that he had arrived at independently. He saw this as evidence for the deep truths in his own thinking.

Whatever the true genealogy of the ideas, I think there is a considerable convergence of thought in trying to solve the same problems.