Even simpler than that.
It means the legislative branch, well legislates. The executive branch simply can't make up rules and fines because they want to.
If congress wants to give EPA this power, fine. Give them the power. EPA (and other agencies) can't simply say "We were given power to regulate X, but since Y is close we'll take it upon our self to regulate that too."
As for diesel emissions @Cat sdp it's unlikely. Most (all?) of those standards were passed by congress so are fine under this ruling. For example "The Diesel Emissions Reduction Act of 2010" and "Diesel Emissions Reduction Act of 2019" among others.