This sounds about right... I suspect Irish Ago is correct. I had to do something similar in Australia a few years back for a race at Bathurst. I handed over my ACU license in exchange for an Oz race license and swapped it back at the end.
I think there's some sort of FIM rule that says you can only hold one FMN license at any time and, if it's the Mountain Course, that would probably need to be an ACU license (- correct me if I'm wrong, MCUI riders).
As long as the ACU recognises the organising body for the rider's nation, it should just be a straight swap for the duration of the MGP. Some FMNs don't allow their license holders to compete on the Mountain Circuit (- Spain, I think) so I've heard of Spanish racers presenting Mexican(?) licenses!
It's all very confusing...;-)