There's certainly a ton of circumstantial evidence like Biden's lawyer asking for the hard drive back and nobody denying it's real.
I imagine the candidate himself is not denying it, because even addressing it to defend himself from a totally false accusation turns it into a headline. Which, given the content of these leaks, is obviously the entire point of them.
[0] https://www.politico.com/news/2020/10/14/biden-campaign-lash...
This seems to dodge the actual matter at hand. The question is whether the email conversation was fabricated or actually happened, not whether everything the participants said in the conversation was true.