Cumming on underwear doesn't implicate one who flashes or urinates in public. If that was the only evidence in the case it would have lead no where it would not even be able to justify a trial. The main reason he was convicted was because of his confession. His defense did a horrible job in the suppression of the confession and worse by missing evidence that made it impossible for him to be the criminal namely an alibi and the fact that he was infertile so it can't have been his sperm. The girls lies should have meant nothing and he should have been acquitted if he had a decent defense. What they claimed and what Julie claimed are not comparable at all.
Of course the claims are not comparable.
I identified a case where four 13 year old girls, all friends, provided false testimony at a criminal trial. They lied simply for "a laugh". The defending QC, David Waddington, was unable to expose their lies during cross examination. The assumption that if prosecution witnesses lie the judicial system will expose them by careful and skilled cross-examination by highly trained and experienced QC's is wrong.