I don't endorse the use of third-party tools, but it works the other way around too. If you have 0 achievements unlocked, then your "number of 100% completion games" isn't affected either.
That's something important to consider, as Unturned is a live-service game and we plan to continue supporting it (which includes new achievements).
(I've spoken with Nelson about it in the past, as this is something 100% completionists have complained about before. I'd personally love for us to include an in-game achievement menu for *resetting** your own achievements, which would technically allow for the above considerations to occur in an official manner. But this isn't an idea we've discussed further.)*
Regardless, I saw guides/mods posted just a few hours after the achievement was added. :x