This has implications well beyond just the EPA, and to the foundations of the “Administrative State”, if regulators are now limited to what congressional law says, and anyone can file suit against regulators claiming that their regulation is nowhere authorized by statute.
You can file endless lawsuits now, gum up the works, and have the regulators stuck in court for years. You’re right, this ruling has ramifications in many areas.
Deep State, Administrative State, not much difference. Needs to be brought down to size! The People rule, not unelected bureaucrats.