You Cheated.
The Dome surrounding the arena might have covered the events within from the sights of anyone attempting to snark a peek but it didn't do well to hide the sounds resulting from the battle within.
Plus the battle was causing tremors all about the arena for many miles over and there was no way anyone would not sense it.
It particularly became predominant after Hal stabbed Elenor with his trident as it was from that moment that the battle reached its height in intensity.
While the attention of those in the meeting hall who had been charged with deliberation was surely caught, they did not make their way to the arena to be spectators.
If their chieftess wanted their presence, she would have made that wish known... Since she did not, they took this to mean that ongoing proceedings were a private affair.
Plus, the tremors they could feel were in no way something they expected to feel from a battle in which Elenor the Malignant was actually serious.