We do know, at least if we take the accuser's word as true:
> A support agent followed up and asked for a copy of Doe’s ID so they could prove it was him and after the teen complied, there was no response for a week, the family claims. (...) Finally on Jan. 28, Twitter replied to Doe and said they wouldn’t be taking down the material, which had already racked up over 167,000 views and 2,223 retweets, the suit states. “Thanks for reaching out. We’ve reviewed the content, and didn’t find a violation of our policies, so no action will be taken at this time,” the response reads, according to the lawsuit.
We still don't know what Twitter knows. It's not even clear if Twitter gave a more comprehensive reason.
That said, if we assume what the accuser said and implied is true, then Twitter was criminally negligent IMO (IANAL). But I guess we'll find out in court.
> A support agent followed up and asked for a copy of Doe’s ID so they could prove it was him and after the teen complied, there was no response for a week, the family claims. (...) Finally on Jan. 28, Twitter replied to Doe and said they wouldn’t be taking down the material, which had already racked up over 167,000 views and 2,223 retweets, the suit states. “Thanks for reaching out. We’ve reviewed the content, and didn’t find a violation of our policies, so no action will be taken at this time,” the response reads, according to the lawsuit.