The Constitutional Convention was originally intended to revise the Articles of Confederation. At some point, the framers decided to start from scratch instead, and produced the Constitution, which fully came into effect when ratified by 13 states in 1790.
For the most part, the Constitution of 1787 covers the same topics as the Articles of Confederation, and obviously the Constitution takes precedence in case of conflict. However, nowhere in the US Constitution does it mention the Articles of Confederation, nor does it explicitly state that it supersedes it entirely.
So, what, if anything, would prevent Canada from hypothetically demanding to be admitted as the 51st state, as stipulated in Article Ⅺ?
Canada acceding to this confederation, and adjoining in the measures of the United States, shall be admitted into, and entitled to all the advantages of this Union; but no other colony shall be admitted into the same, unless such admission be agreed to by nine States.