Fowler's post established that this was happening to many women in the company. If we accept that Fowler is a credible witness, which I think is a fair assessment, we already know that this is a systemic problem. It's safe to assume then that if more women come forward, they are more likely to be telling the truth than not.

Additionally, it's in Lyft's or other detractors best interests not to fake additional claims. This is a scandal that will grow on its own, as/if more women come forward. If Lyft et al is caught faking claims, they discredit all the claims and make themselves the bad guys instead of Uber. All Lyft has to do is sit back and watch Uber dig themselves deeper into the hole they're in.

As other posters have also touched on, there is a history of men (and women) doubting and minimizing abuse claims made by women. This is not a good mindset to take as it minimizes the women's experience in favor of a corporation or individual who is already in a more powerful position than the accuser. By minimizing one accuser, we make it harder for women as a whole to speak up against abuse, which is not acceptable.