There are many reasons why an assault victim may not report it immediately after it occurs

The assault could seem minor, i.e. "it's not r*pe, so it's not worth reporting". The victim could figure they wouldn't be believed if it's their word against another's. The victim could reason that reporting it might jeopardize their career or their reputation. Reporting it right when it happens is no guarantee that you'll be taken seriously either. Additionally, when one person reports it and their story gains notoriety, other past victims of the same perpetrator may be motivated to report their experience as well, even if it is long past when it happened.

None of this guarantees that their accounts are true. But it certainly doesn't mean they've lost credibility.