Imagine Alice is selling Bob a house. Carol comes along and says, "Don't buy that house. It's infested with termites." This is likely to cause Bob to reconsider his offer, and Alice will be somewhat pissed. If Carol is correct, there's nothing Alice can do, but if Carol is lying, Alice will likely sue for unlawful tortuous interference with business or somesuch, whatever the legal words are.
Now imagine that Alice is selling Bob a Monet painting. Carol says "That painting is fake." If Carol is some internet rando, probably doesn't matter. But if Carol is well known among Monet collectors, then her statement is likely to dissuade potential buyers. Alice will lose lots of money. If Carol is right, end of story, but if Carol is lying, perhaps to spite Alice, then hello lawsuit.
Now, getting closer to the case at hand, imagine Alice is selling Bob a Peter Doig painting. Carol says "That painting is a fake." Same story, if Carol is lying and that depresses the market price, Alice has a case against Carol.
In this particular case, it turns out that "Carol" is actually Peter Doig himself. But that's somewhat irrelevant to the legal framework. Actually, it just means he's considered an expert, and therefore his statements will affect the price. But being an expert doesn't make his statements true.
Of course, with evidence presented, obvious he's not lying.