A recent New York Times article wrote this about the US state of Maryland.

Zachary Clopton, a law professor at Cornell University, said Monday’s lawsuit may be more serious than the New York complaint because Maryland is legally considered a “coequal sovereign” with the president. That makes it a stronger opponent in the constitutional argument over emoluments, he said.

What does "coequal sovereign" mean in this context? And does it apply specifically to Maryland or to every state?