The source for the graph is just "NASA", without even a hint of a proper attribute. The link to the sciencedirect paper is a nifty and super geeky description of a continuous effort to extract more and better information from one of the many, many datasets NASA has on wildfires.

Without a proper citation, climate change denier is denying climate change in a way that doesn't have enough data to challenge. Given that Bjorn is supposed to be an actual scientist, they should know better.