I missed the video embedded in the Mother Jones article. This is hugely funny. The author seems to think that hide the decline was something to do with assessing twentieth century temperatures:
...all the fuss over the decline came from one obscure dataset showing tree ring densities in some high latitude regions. When that data was computed in one specific way, that formula gave scientists the wrong idea about the Earth's climate over multiple centuries, and when they realised this they stopped using that formula on tree ring data to look at temperatures after 1960 and relied more on things like, say, actual recorded temperatures...case closed.
Ye gods, even the Hockey Team guys aren't arguing anything so daft. Mother Jones, if you're listening, the point is that the Briffa series doesn't track temperature - it declines while instrumental temperatures are going up. If tree rings (at least some of them) don't track temperature nowadays, it is not possible to use them to recreate temperatures of the past. The point about hide the decline is what it tells about what is knowable about medieval times, not about modern temperatures.
Hilariously, this video came to you via one of Mother Jones' factcheckers!