It's not a thought experiment; it's literal, to an extent.

Within the 'god is dead' conceptual framework, Nietzsche needed a post-christian metaphysical formulation - eternal reoccurrence was his best shot at that.

I specify "to an extent" because Nietzsche's reliance on the theory in his work was limited by his inability to make it logically bulletproof, but he really did believe in the idea of a cyclical universe.