It means "concerning that about which we were speaking," or just plain "about."
Other than that, and a few typos, which nowadays seem inevitable, this is a hard-hitting piece of legal writing. The hook is well baited. Now let's see if the judge bites. Or bites back.
I think the legal eagle has used the word apropos appropriately and in a pertinent manner. I understand what he was saying.
...except that it is totally wrong. The defense did not have to file a Notice of Cross-Appeal to raise new grounds, because an appellate court has the power to affirm a judgment of dismissal on any ground found in the record, even if it was not the ground relied upon by the trial court. (In other words, if the trial judge reached the right result for the wrong reasons, the Court of Appeals must still affirm.) Perkins Coie did nothing more than raise additional arguments why the district court's dismissal of the suit was proper.