OK, so basically, the plaintiffs here are arguing that the US Department of Agriculture which originally used its authority to put the rules in place that provided these loopholes for illegals all of a sudden now lacks the authority to change those same rules.
If they didn't follow the requirements of the APA, then the action rescinding the benefits may be subject to being overturned.
This is a perennial failure of the Trump administrations - a complete failure to appreciate both the benefits of using, and the risks of ignoring, the APA.