To say the "new evidence surfaced from Warner Music" is rather misleading. The plaintiffs independently found the evidence; what they got from Warner had the evidence "blurred out". Here's the summary from TFA:
"(1) Warner/Chappell Music (who claims to hold the copyright for the publishing, if it exists) suddenly "found" a bunch of relevant documents that it was supposed to hand over in discovery last year, but didn't until just a few weeks ago, and (2) a rather important bit of information in one of those new documents was somewhat bizarrely "blurred out." This led the plaintiffs go searching for the original, and discover that it undermines Warner Music's arguments, to the point of showing that the company was almost certainly misleading the court. Furthermore, it definitively shows that the work was and is in the public domain."
Warner, of course, denies that conclusion. rsilvergun may be right, but the date of the songbook relative to the date of the "copyright" and of the changes to copyright law would seem to weaken Warner's argument fatally.