You see an anomaly and you try to explain it. It does NOT discredit the theory of AGW (as global temperature averages have certainly increased), but the causation may have changed over time after a threshold amount of CO2. The article, which I have yet to read, cites possible additional causes such as solar radiation, cloud formation, and natural temperature cycles, something that their work has included.
But, I suppose more than a two word answer is needed here.
First, the problem in this latest news cycle with the BEST work is that the talking point has become Global Warming = the surface temperature record. That’s way too narrow of a view of what is really happening on the Earth’s surface, and as I’ve mentioned before simply taking the surface temperature record isn’t even the best way to understand what is really going on (for which one really needs to delve into the energy flow from the surface into space, and realize that changes are taking place from the mid ocean depths up through the stratosphere.)
Even then, taking a surface temperature record including the polar regions (which the UK MET office does not!) does show surface temperature rising over the past 10 years.
As for “causes” - they are the same as stated before, and have been known (generally, and in many cases specifically) for many years and continue to be re-examined in more detail. The least well understood appears to be cloud formation. One of the best understood is the “greenhouse” effect of gases in the atmosphere. Ocean heat transport is something that is also understood in general but has many details in the specifics that are being investigated. The affects of the biosphere also comes into play. In this latter case human transformation of the landscape is of particular importance. And, as for incoming solar radiation - it too has been well studied.
Overturning of the ocean (via currents), biosphere changes, aerosols, and the solar (psuedo-)cycle are more than enough to account for the year to year variations. Indeed, it was only about two years ago scientific papers showed up warning about a few years of North Atlantic ocean heat absorption (thus leading to less surface temp warming) around the early 2010’s (I think Susan Solomon was involved in some of this - you can Google her name for more details.)
Variations from year to year in the surface temp record, and even within a whole decade, are to be expected from a system as large and complex as the Earth’s surface.
Judith Curry doesn’t want to give up the spotlight, and Muller seems content with the idea that he’s made his splash (see his books). That’s what is going on here.