Here are some of the top answers to this seemingly random question. All based from /r/NoStupidQuestions:
I think I asked this at the time and a family friend said they had moved to a temporary office. Most of the employees weren’t at work when the first plane struck, so there were a lot of survivors.
Those companies have temporary and satellite offices all over the damn place. Often they have the space to house quite a lot of overflow were it necessary.
That’s what happens when you’re insanely wealthy investment and banking firms.
They are called “disaster recovery centers,” incidentally. And they were around well before 9/11. As a market data service technician I visited one or two of those. Also years later I visited one for a major telecom provider, so it’s not just banks. This kind of facility will basically have one of everything they have at the main office. Just in case. And they probably have more than one of these facilities. Just in case. They spend the wide majority of their existence only sparsely occupied, mainly just the technical staff that makes sure everything is chugging along.