Historical question

[Chandran, Nanda (NBC)]

Time and again, in various books, I've read that when the Brahmins got
too deep into ritualism, the Kshtriyas to offset this with rational
thinking, authored the Upanishads.

But since the Upanishads are themselves but part of the shruti, which
itself is termed apaurasheya, how can this claim be valid?

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