According to the article, the ballpark is 2 weeks of engineering, so it is probably done in less time.
Why so quickly? Because the user used an extremely weak password.
Security is also the responsibility of the user, not only the tech companies. If you are really concerned with your own privacy, than you shouldn't encrypt your data with a 4 digit passphrase which is relatively easy to crack.
Considering the commited crime, and the relative ease of cracking the phone I'm in favor of the FBI. As long as the firmware is not handed over to them, but that should be possible I presume.
People worrying about Apple engineers smuggling the software outside should be worried everyday. People with access to the codebase can do this probably allready.