My master and teacher the great Gaon R' Asher Weiss, May he live and be well, points to many difficulties with this interpretation. First, we have no reason to believe that the Rema ever saw the book of Alqabets (Manot Halevi) who was his contemporary. Second, even had he seen it, the very idea that the Rema would have based his Halachot on the book of vertlach of (who with all due respect was not quite on the level of the Terumat Hadeshen) Alqabets is unthinkable. In general the Rema focuses on the teachings of the Ashkenzim (such as the Terumas HaDeshen) and he would be unlikely to base himself the interpretation of Alqabets who was a Sephardi. Besides for all this, this isn't the correct explanation of the Passuk, according to the Peshat. The simple interpretation is that Haman was telling Achasverush that the destruction of the Jews would not cause a specific land to be devastated since they are spread out in many lands. Certainly within the seventy sides of the Torah, each side of which the Gra explains has 613 gates there is room for this explanation as well but it is not the plain meaning of the Passuk. (Note: that the Bach does make use of this Manot Halevi but it is difficult to explain this to be the understanding of the Rema.)
Further, one needs to explain an apparent redundancy in the Rema's language, he refers to his friend "not wanting to accept" and "forgiving" the gift. If one doesn't want to accept then obviously you are forgiving and so vice versa. What is the difference between these two categories?