Nietzsche actually thought Darwinism to be true - and a terrible truth at that.

Nietzsche [like many today] slightly confused Darwinism itself with the ideas of Herbert Spencer [who actually coined 'survival of the fittest'].

Nietzsche quibled that evolution [an idea he accepted as it was first mooted by the Presocratics] was aiming at survival. He didn't think evolution was aiming at anything else other than evolution itself; and as for 'survival', he thought that life strove only to overflow and flourish - The Will To Power.

In all this, Nietzsche wasn't that far off from Darwin and Darwinism. This is why Nietzsche was taken up by philosophically minded Darwinists in the early 20th century.

How anyone cannot see the evolutionary dynamic to a philosophy of Becoming, Will To Power and the Superhuman, is beyond me!