You can go longer with twins. This is about elective deliveries that are about nothing other than convenience to the mom/doctor/etc. It is not about early labors that cannot be stopped or anything medically necessary. There will always be medically necessary early deliveries.
I think what all of the recommendations are about are preventing issues in our babies if at all possible. Twins is a great example because most doctors will have said that you need to have them delivered by a certain week- but why? If babies are doing well and so is mom, they do not need to be induced earlier than a singleton.
The medical community is of course aware of this information and is supposed to be acting accordingly. 39 weeks before induction unless medically necessary. Induction is different than just having your baby naturally because baby is ready to come early. One of my kids was 10 days early and she is just fine. She came when she was ready!