The only time I've heard of the idea of "co-equal sovereign" is within the doctrine of Dual Federalism. As suggested by this textbook it refers to
A system in which the states and the national government each remain supreme within their own spheres. The doctrine looks on nation and state as co-equal sovereign powers. Neither the state government nor the national government should interfere in the other's sphere.
However I don't believe this practically true any-more, as the Wikipedia article suggests:
The general consensus among scholars is that dual federalism ended during Franklin Roosevelt’s presidency in 1937
In fact most historical cases (at least in the last 100 years) have recognised the supremacy of the federal government over individual states, and I cannot find any other legal reference to whether Maryland (or any other state) any longer enjoys co-equal sovereignty.
The wider point the professor is trying to make, about a State filing a case against the President being more serious than other non-government actors filing such a case is however valid.