They DO state that they are taking on new staff AND reprogramming the whole thing.
This makes me thing 24 hours was a massively low estimate, and they just couldn't get the work done. How many I.T. projects don't come in late? ..answer is, almost all of them, especially those with 0 fault requirements.
You hear what you say? Good development not need to shutdown the live site. They not have a test environment and when the new code is ready and tested just will update to the live server? Of course it is possible to stop the site for a while but not so long as you try to say.
You don't keep a live site up and running when you have potential for massive theft. In light of the Gox fiasco I have to imagine thats some part of what they were looking at. ..and that is eluded to in some of their earlier posts. Taking it offline is costing them money, and reputation as you see here. Its actually to their credit that they were willing to do that in order to safeguard the coins they have.
It would have been far easier for them to do as you say. They could have just kept the site running, and developed the new engine in the background. This would of course mean leaving all of us blissfully trading while coins were potentially being siphoned off by exploit. Potential there is they go insolvent just like gox and find themselves unable to pay any of us.
Having a test environment is really unrelated to that. They almost certainly have one, but it would be impossible for me, or anyone outside their team to know the details of that.
I know we live in the generation of panic and instant gratification, but a few days is not so long.