This was the first time that the data was available. As McIntyre points out in the top of his post, the decline was discussed in the 1998 and 2001 Briffa papers.
Of course, scientists could not actually evaluate this because the data was not released. But that's not the issue.
The issue is, as McIntyre also points out, that the IPCC 2001 graph (and later graphs) simply truncated the climate reconstruction data in 1960 and pasted the (more accurate) direct temperature station measurement data onto the graph after 1960.
While this was done it plain sight, it's still lousy scientific practice. Why? After all, shouldn't more accurate data be used? The problem is that comparison of temperature reconstructions with direct measurement tells you a lot about the accuracy of your reconstruction.
In this case it tells you a lot about the inaccuracy of the reconstruction and the sorts of signals that are missed.
I'm sorry, the paleoclimate reconstructions available in the literature are not convincing. They often use dodgy statistical techniques, mixing data gathering steps with data correlation steps to increase apparent agreement with the instrument record. There have been huge problems with releasing data and there are often bizarre choices made in proxy record selection.
Without good paleoclimate reconstructions, it's hard to know how far outside (or inside) the historical standard deviation for temperature we are. I happen to think it's an important question to have answered. A trillion dollar question.