If you have realized that all phenomena are nonself, why are you asking this question?
"Did Siddhartha Gautama achieve Nirvana?"
No, he didn't.
Now you're thinking "If he didn't achieve Nirvana, why every book says that he achieved Nirvana?"
If you have realized that all phenomena are nonself, Why are you asking this question?
"If he didn't achieve Nirvana, why every book says that he achieved
Nirvana?"
Most people cling to the idea "self exists". Most people are not yet ready to realize that all phenomena are nonself. Most people in their current lives will not realize that all phenomena are nonself. If you were to speak to them from the perspective of anatta, they would be confused and bewildered. Most people think that all phenomena are self. That's why the books are written in such way, so that most people would understand them. If such a book is written from somebody who achieved Nirvana, it is written in such way purely for others to understand it easier and to guide them in the right direction towards realization of anatta.
"So instead of asking who attained Nirvana ? Can I ask the following
question : did Siddhartha Gautama achieve Nirvana?"
"Who attained Nirvana?" and "Did he achieve Nirvana?" are both the same questions.