Reloading the UI only reloads the UI. Since the overhead text that shows HKs is not rendered as part of the UI, it's not affected by reloading the UI, and it won't pick up on changes to the DAMAGE_TEXT_FONT global during play. You'll need to log out and back in to apply any changes.
Removing the last line in that code makes it stop working, because you have to set DAMAGE_TEXT_FONT in the main chunk. If you wait for ADDON_LOADED, you're probably too late. If that's the entirety of the addon, then it can be stripped down to a single line of code without losing any functionality:
Code:
DAMAGE_TEXT_FONT = "Interface\\AddOns\\gdFont\\Fonts\\SKURRI.ttf"