This whole taking the store offline thing is not because they need to, but because they want to.
Of course an IT company like Apple is capable of updating it's store without downtime.
They use this gimmick purely to draw attention to the event.
I'm not sure if it's entirely a gimmick. Apple has something that survives under an absolute deluge of pre-order traffic. Look at how the likes of Microsoft coped with Xbox Series X/S pre-orders. Their stores were constantly crashing, as were many online retailers who were acting as resellers. And there's no way they were flooded with the kind of traffic Apple gets when launching a new product.