In this case I think the evidence is actually Uber's lack of rebuttal (admission by omission). For example, searching the slack logs on the first day of Susan'

In this case I think the evidence is actually Uber's lack of rebuttal (admission by omission). For example, searching the slack logs on the first day of Susan's employment should literally take less than a minute. If there was no proposition there, Uber would have made a press release stating exactly that a day ago. They haven't. The logical conclusion here is thus that the evidence you want exists and Uber isn't (and probably won't) releasing it to the public. Put another way, what guarantee do you have that Uber will actually provide the evidence you're seeking one way or the other? On one side we have a claim made by a former employee, backed by "that doesn't surprise me" anecdotal stories from current and former employees, and on the other side we have... a promise to investigate internally using a highly biased team. It seems to me you're kind of looking for a courtroom-level burden of evidence that simply isn't realistic to expect in this situation.