Short memory. Henry took over an All Blacks team that had severe culture problems, poor player attitudes and a less than stellar record in big games. HE changed all of that and rightly deserved his accolades. The All Blacks in 2003 were a team of talented players, but they weren't the most tactically astute of sides and the had a soft underbelly with plenty of poorly managed egos, and alcohol problems to boot by all accounts.
Fixing that up was a big job, and Henry did it marvelously and it was a credit to the NZRU that they could see that in spite of their shock 2007 RWC exit.
So it's not about it being the All Blacks - it's about the amount of work that was necessary to achieve what they have. I think you're talking this as some slight against the All Blacks, but it's not. It's just a simple statement of fact to say that it is more impressive from a coaching standpoint to take a team of underperformers and have them be contenders, rather than take a wildly successful team with few issues and continue that success.
Hansen may well be capable of replicating that elsewhere, but we don't know that yet. Everyone thought Deans was a genius too until he coached the Wallabies.