He didn't call anyone a liar at all...
Thing is, no matter how much we believe or don't believe or how angry/sad/dissapointed/disgusted it makes us feel, when people are accusing him of harassment - and it's so far only accusations - it isn't unfair to ask for evidence when people are accusing.
Usually it's very difficult to prove anything (for example if it only happened face-to-face), but people in this case are referring to concrete DMs here. They've already said A, they should say B to end any doubt that might be.
Of course it looks really bad for him now, with so many chiming in with their own confirmations, and it looks like this is all true (and all the hate justified). I'm just saying it's not beyond reason to ask for someone to actually provide what they say they have. Not necessarily for everyone to see, but maybe to someone who can verify? People are calling for him to prove his innocence - that's not how it works at all.