I believe this is like some of the other classified area applications, where they want some clear cut definitions of what is classified and what isn't. The extraction equipment can be listed equipment, and they think that much of this equipment will be installed without the benefit of engineering experts being hired to provide the area classification drawings that 500.4 requires.
The code, other than in the specific articles such as 511, 513, 514, 516 and now 512, does not provide guidance on how to classify an area. The code only tells you how to wire the area once it has been classified by experts in the field, and those are typically process and mechanical engineers, not electrical engineers.