Question

how did Siddhartha attain Nirvana

Answer 1

The Buddha achieved nirvana, yes. I think all Buddhists would agree.

Supposing that you definitely mean "Guatama" to refer not to a person but a sequence of conditioned phenomema, I suppose you can say that, though it might imply something like buddha nature, that his enlightened nature existed before it was attained.

Answer 2

While meditating under a pipal tree, Siddhartha attained Nirvana. He attained Nirvana at the age of five