Actually, we can assume a couple of things...as he attempted to work out a plea agreement. He agreed to plead guilty to sex with a minor, face some sort of prison term (supposedly short) and deportion. His guilty pleas says he had "sex" with the minor...and that legally, because she cold not consent, it was a crime. We can also assume that while mutually agreed to by the State and Polanski, it was done so in order to avoid a trial. The trial likely would have been on the forcible rape, drugging, kidnapping, abuse of a minor. We can judge to some extent what Polanski thought of his chances in a criminal trial by his taking the deal. While we can not know how strong the State's actual case against him, we can judge that Polanski understood that going before a jury that has heard that he had an unsupervised 13 year old over for a hot-tub party would pretty well go South for him from there. Afterall, how could it not? By every standard I know, a 40 year old man entertaining a 13 year old, alone, at a hot tub part looks at best creepy (because it is), than when you add sex in, it is really ugly. He knew that if he was judged by a jury on the same charge that he plead to, the Judge would throw the book at him...years in jail. How could the judge not? He was a 40 year old man knowingly "entertaining" a 13 year old. It shocks the consceince. So he plead guilty, not because he was even close to innocent (he was not innocent) he plead to try and avoid real punishment for his actions and he fled to avoid responsibility.
My point is, you can get into the Whoopie issue that somehow statutory rape is not "rape rape" but ultimately, it seems to me it is meaningless. Even in a he said, she said situation where he alleges "consent" and she says she said "no" -- in other words, take the "statutory" element out of her ability to consent -- and you are left with a rich, powerful, 40 year old man and a 13 year old girl. Believe me, even in the permissive 70s, an LA county jury (not made up of Whoopie or Woody, but pipe-fitters, secretaries, and school teachers) would have found him guilty. He knew it.
And, don't assume that the State didn't persue it because they didn't have evidence, as is true with many a rape (statutory or otherwise), the State does seek to minimize the impact on the alleged victem. In short, one motivation for the State wasn't the weakness of their case but to keep the victim off the stand -- and, given her age, they were, IMO, right in doing so.
So, from the circumstances, you can pretty well be assured that it was "rape rape" if you want to try and make a meaningless distinction between that and a statutorilly illegal sex act...