This was a movement for summary judgment which requires a very high standard: they are basically responding to a threat of a lawsuit by asking a judge to declare in advance that this lawsuit can never truly happen; effectively they are asking a judge to affirm that the threat is really, truly BS.
They gave a lot of reasons why the lawsuit might be invalid from the get-go, from "how do we know that the purported author was actually the first to write this?" to "how do we know that she didn't abandon her copyright interest before the critical date?"... but for a summary judgment, you don't need to show what's true but rather what's arguable. The judge denies the vast majority of these complaints by saying that even though the plaintiffs might have given some good evidence that those things are what happened, they didn't show that it was inarguable.
The sticking point is that the copyright today is only on the words, not the melody and various arrangements, which have long since passed into the public domain. However, according to the judge, there is absolutely no evidence that the copyright for the words was ever transferred to that company alongside the words. References to the various lost agreements between the parties generally explicitly say that they were agreements to transfer the music, and make no explicit mention of lyrics; and the lawsuits that the company began litigating similarly complained about the use of melodies or arrangements rather than complaining about the words being used. On this one narrow point the judge said, "no, that's inarguable: there is no argument that in all of this evidence you've brought me, that there is nothing transferring lyrical rights to the company.