Why would they offload passengers to carry extra fuel when they could just refuel in GVA?
I think you're missing the point.
Firstly, I agree with you that we're probably not talking about a ZFW limitation here.
The way I read things is that the fuel loaded, given the originally-calculated passenger weight, produced a take-off weight of just under MTOW. Nothing wrong with that, of course.
Then someone does a few fresh sums and concludes that the unusual male/female pax split has resulted in a higher take-off weight than orginally calculated, that now exceeds MTOW.
The fuel is
already on board by now, so my original question stands: which is easier to offload - pax or fuel ? No contest.