You are correct that it is contained in a legal brief.

However, it comes directly from the Administrative Record, which is referenced at least 4 times in the course of that brief.

The quoted section is the only instance of these 4 where the Administrative Record is directly quoted, as indicated by the quotation marks in the unredacted sealed brief as filed with the Court.

The TSA did actually state "terrorist threat groups present in the Homeland are not known to be actively plotting against civil aviation targets or airports".