The power grab that was done was with Congress abdicating their power into unelected bureaucracies to create and enforce rules with the force of law. What the supreme Court is saying is that Congress cannot abdicate their power into an unelected bureaucracy. Congress can pass laws and the president can have people enforce those laws but those laws cannot be so broad as to say this unelected enforcement agency can literally write new laws in the form of regulations and then enforce them as if they were law without any kind of recourse. That is a fundamental violation of due process to say some agency was empowered with such broad authority and such broad scope that they can literally write any rule that they want and you must follow it.
To limit the agencies to actually enforcing the law that Congress passed and requiring that those laws be specific enough that they are not open to broad interpretation and redefinition is the opposite of a power grab. It is the limitation of unelected bureaucrats in their power. If Congress wishes to re-empower them they simply need to pass the laws. Those experts, as they claim in the article, have within their power to suggest new legislation for things they want to implement. Having them just be able to arbitrarily make any new rule that they want under a broad umbrella of you are empowered to protect the environment is akin to a dictatorship.