Many games in Early Access (EA) receive a sudden boost in player activity after they move out of EA. Satisfactory is one recent example. Their 1.0 release boosted both sales and existing player activity tremendously. Of course, one thing they did was to keep a huge part of the game a secret that was only integrated into the final 1.0 release which meant none of those features were tested in EA. Farthest Frontier has exposed their code to modders in a way that I expect many new features to come out of their mods should the developers decide they want to incorporate them into the game.
I came to the same conclusion that these 1% or less achievement rankings were simply a sign that most of the players have not returned to the game since achievements were released, which was fairly recent. Give it time and especially after leaving EA, these achievements will receive an up tick in statistics. There are still many achievements we cannot unlock due to features not yet in the game. Some players may just be waiting for them to be included as well.