There is never a mitigating circumstance for rape.
This is an important point. Obviously there are mitigating circumstances for theft (eg someone might be impoverished, or they might be coerced by a gang-leader), and there are even mitigating circumstances for serious crimes such as assault or murder (someone might be unduly provoked or they might be a victim of abuse acting in self-protection) but I can't think of a single mitigating circumstance for rape.
And even if these boys have a low IQ, they knew exactly what they were doing. It was a vile, pre-meditated crime.
The message this sentence sends is that men can rape with impunity. We knew that already, of course - most rapes don't get prosecuted, and those that do often don't result in conviction. But now we know that even if you are convicted you can expect a slap on the hand and some warm words about what a nice person you are really.
Meanwhile, the poor victim who has been through the unbearable trauma of being raped, followed by the additional trauma of having to relive her experience in court, in front of her assailants, counts for nothing. Her feelings, her distress, the fact that her life has been ruined, are of no consequence.