Here you have another new innovation in this Megillah which is not part of our Massoretic tradition. The fact that the Shem HaShem doesn't appear in the Megillah doesn't deter this scribe from highlighting any allusions.
In this case the sofer has highlighted the "Rashei Tevot" H-V-H-Y in order to bring to mind that HaShem is "hidden" and working in the background. The fact that the letters are in reverse order is most important, for one can only be cognizant of the YAD HASHEM when looking "backwards" and examining all of the events after they have taken place.
In a similar way, we have also done the same by adopting the modern convention of initiating many of our columns with the word HaMELECH. This is our way of bringing out the same concept that, in the story of the Megillah, HaShem is hidden and working in the background. Therefore, we interpret that whenever HaMELECH is written by itself - this refers to HASHEM; but, when haMelech Achashverosh is written - this refers to a king of flesh and blood.