SuperBeagle - You seem to have missed the basic fact that this man was convicted of the offence. Your statements about the law are wrong.
The fact he genuinely believed the girl was over 16 is not a defence. This is a strict liability offence so mens rea (guilty intent) is not required for the defendant to be found guilty. The only available defence is that the accused did not have penetrative sex with the victim. If she had been over 13 the law does allow a defence that he thought she was over 16. That defence is not available if the victim is under 13 as in this case.
However, the fact that everyone who saw the victim that night appears to have thought she was substantially older than her actual age is a mitigating factor which the judge has taken into account, resulting in an absolute discharge. Note, however, that the OP isn't entirely correct in saying he got off scot-free. He was convicted. This conviction is on his record and will appear on any DBS checks for the rest of his life.
This trial took place under Scottish law which is broadly the same as English law for this particular offence. However, there are currently no sentencing guidelines in Scotland. The guidelines in England suggest that this offence would have fallen into category 3B (lowest level of harm, lowest level of culpability) so the normal sentencing range would be 6-11 years. However, the courts are able to step outside the guidelines in exceptional cases so it is not impossible that he would have received an absolute discharge if this had happened in England.