Good one, keep telling yourself, and everyone else, that, Repeat it often enough and it will be true.
A wisecrack is not a response. And it is true. The Maryland legislature did not consult with any other state or enter into a compact to do this. That the law only goes into effect if similar laws are enacted by others does not make this an agreement in violation of that clause of the Constitution.
In fact, you are seriously stretching the meaning of the word "agreement" in that clause, which doesn't even cover informal agreements between states (not that this law is one of those, either). The only thing it prevents is an enactment of such an agreement in state legislation, e.g. if California and Oregon were to pass laws requiring each state government to buy goods and services only from the home state or the other state.
Here, look at this: "The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution,
or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress . . ."
U.S. Constitution, Article V (emphasis added). Now I ask you, how could the state legislatures of 2/3 of the states call for a constitutional convention without some degree of informal collusion among them? Obviously, then, that kind of ad-hoc and de-facto cooperation is not what the clause you referred to is intended to forbid.