Samkhya - the Upanishads?

[Chandran, Nanda (NBC)]

I'm reading S.Radhakrishnan's translation of the Bhagavat Gita and in
the second chapter dealing with the Samkhya thought, he says it's not
the school of thought as taught by Kapila, but the Upanishads. I
remember Vidyashankar, in a previous post, saying that the Gita borrows
the varna concept from the Samkhya 'school' of thought, which led me to
think he referred to one of the six schools of Hindu philosophy.
Though it's condisdered as one of the the orthodox schools, how would
'Samkhya thought' refer to the Upanishads? Can the knowledgable explain?

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