alt Hacker NewsYour assumption is correct, they were prohibited:
>3.7 Heavier than air: The UAS must be a heavier-than-air aircraft. The use of any lighter-than-air gases to provide buoyant lift is strictly prohibited. Teams may use chemically inert gases for subsystem actuation, safety inerting, and component rigidity, provided that there is no buoyant advantage and the gas is contained in components operating at pressures indicative of mechanical function.
Someone did fly a sort of balloonocopter, a quad copter that used inflated balloons as the cross arms, My disappointment is immeasurable that they were not allowed to use a lifting gas in it. I mean, it flew poorly anyway, but seeing as it is an unmanned vehicle, I would have used hydrogen as my lifting gas, and then when it had a catastrophic failure it would get a viking funeral for free.
As it is, there were some impressive metal fires during crashes, another win for lithium batteries.