> I don't quite understand what they're saying here...I guess the idea is that Congress can regulate gambling -- assuming some constitutional justification like the commerce clause -- but they can't restrict what laws states pass?

The federal government can make it illegal for people to place sports bet or run a gambling ring, because it has authority and sovereignty over the people in the United States.

The federal government has rather wide authority. For example, Congress could make it illegal for American citizens to gamble in Canada (In the same way, it used to be illegal to do business with Cuba). However, Congress cannot pass a law saying that Canada cannot legalize gambling, because the government of the United States has no authority over the sovereign government of Canada.

Similarly, the federal government can make it illegal for its residents to gamble. However, it cannot exercise authority not directly given to it over the state governments, because the federal government has no authority over the sovereign state governments. The state governments and the feds share sovereignty over the boundaries of each state, but the state government retains all aspects of sovereignty not explicitly given to the feds. Gambling is one of those things that the states retain sovereignty over.