This is complicated because the transaction occurred in Canada but the case was heard in Chicago. But the argument is the same as forgery: the court may not be able to force Doig to authenticate it, but if the officer can prove Doig sold him the painting under the pretence of being his then Doig has sold him a forgery (selling a claimed Doig which truly is not) and is liable for it. It does not matter that the name Doig did not mean anything back then, because the forgery constitutes a loss of future earnings.

Of course this is all hypothetical because Doig obviously did not paint this given what has come out in court.