I did one Google search to see if this is true, and found a Politico story that says Biden staffers, after reviewing schedules, denied that the events described in the emails took place. So "no one is denying it" seems false. (Feel free to do the search yourself, it's easy.)

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.