Statute grants absolute right of intervention only to citizens who are barred from filing their own suit to enforce the law due tot he government's attempt to enforce that specific law.
As the government filed suit under the law regulating the devices, citizen suits seeking to enforce clean air laws are not barred, and the potential plaintiffs have no absolute right to intervene.
The present request to intervene seeks relief that is distinct from the government's relief; absent Article III standing (which can't be manufactured by simply seeking absolute compliance with the Act), the potential intervenor does not have an intervention of right.
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/07/03/16-17060.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.