Madison was successful in convinving Hamilton that the resumption of powers at will as declared in the New York and Virginia ratification documents were not conditions, but inherent powers explicit in the Constitution itself.
If that were the case then he would not also have said in that letter that once in the Union always in the Union. It was specifically to put the specter of secession permanently in its grave that he wrote the letter since that was what conditional ratification meant. Hamilton was weakening and almost ready to allow NY to put forth a conditional ratification having been worn down by his titanic struggle against the Clintonians. Madison bucked him up with this letter which destroys any contention that there was a right to withdraw without an amendment.