First of all, you're basically right, the Theater of Pompey was where the senate was meeting that day because the curia had been destroyed. But the senate did not always meet in the same place every time they met. Most important meetings took place in the curia, but quite frequently they took place in any number of locations large enough to house that many people (theaters were popular). During Caesar's proconsulship, for example, the senate had a series of meetings outside of the pomerium, which legally and ritually meant they weren't even in Rome (although of course the city was quite a lot bigger than the pomerium) because Caesar could not enter the city (i.e. the pomerium) with proconsular imperium. The Theater of Pompey was a very convenient place to meet, because as one of the few permanent theater structures it could fit all those people at once and was already set up to accommodate speakers