Good point! I was under the impression that he had admitted that he was guilty in some form. It's unclear to me now if he did. I don't know if anything he's said and done in the aftermath may be considered an admission of guilt. So my previous comment might be incorrect.
It's also unclear if a pending impeachment might be considered a "conviction" if it's just a likely outcome and not an actual impeachment.
[Citation Needed]
I have never seen or even heard of any such confession from Nixon.