No an event planner, so this is an incredulous question rather than refutation... do you really need to reserve the space for 10 days for a 1-day event?
Setup and tear down come to mind. The world works much slower than you think.
OK, but why reserve 10 days for a 3 hour announcement?
Things are different than when Tesla did the S. They will be delivering 10X as many Xs in the first few months than they did Ss and they have 10X as many reservation holders waiting.
Yes, things are different now. Unfortunately your estimates are a little off; there's no capacity to be 10x Model S production, back in the last weeks of Q3/2012 Tesla delivered 253 cars and in the first full quarter Q4/2012 they delivered 2,400 cars. With regards to reservations, Model X reservations are about double the equivalent number of Model S reservations one month prior to Founders launch in 2012.
You can see details of the numbers here.
If it's an Apple event my guess is that it's a Dr. Dre concert to promote Apple Music, the new iPhone, Beats, and his new album/movie. Could be two or three nights with different guests from the album showing up.
It's official, Apple is having an event on September 9th at the Bill Graham Civic Auditorium in San Francisco. This lies within the Sept 4-13th reservation period, so it was likely Apple booking the whole section and they may later revise that booking to a smaller number of days. Or they may use the whole 9 days for prep and take-down.