If I am reading the original graph correctly, it is not indicating a slowdown at all.
This is easy to misinterpret because it shows the anomaly on a yearly basis, which is still increasing.
So, while there appears to have been a sharper year-on-year increase previously, we are still at the maximum per year change.
Even if the graph were to completely flatten out, we would still be dropping per year at the maximum observed rate of ~ 750 k km^2
If you're referring to Tamino's graph, I think it is showing the cumulative anomaly, not the rate of change. So an anomaly of -1 would mean 1 less than normal. not a rate of change of -1 per unit of time.