Because they want to limit the scope of the law to documents only. Why would they question the part of the law they want to keep?

The part of the law they are questioning has to do with actual actions/violence to prevent official proceedings. They are questioning the scope of the other parts, not saying that they intend to exclude it entirely. They can’t make up new laws. They can only interpret them. Yes, they can have poor interpretations, but they’d seriously struggle trying to exclude things entirely without having uproar throughout the federal court system which comprises of several liberal judges as well.