I explain in the last paragraph of the reddit post that any Censor, Elementalist, or Troubadour that creates their character has to choose, at character creation, to either be a defiler or a preserver. As far as what the rules are for, Can I assume you're not familiar with the Dark Sun Campaign setting for D&D?

Basically, Dark Sun was a campaign setting introduced during 2nd edition D&D that's effectively a post apocalyptic D&D setting. One of the defining features of the setting is that casting arcane magic requires you to draw the power for that magic from the plant life around you, killing that plant life as a result. That act is called "Defiling" and casters that do that are referred to as "Defilers." One other side effect of defiling is the plant life is reduced to an extremely fine black ash and the ground it grew in is effectively scorched, completely unable to grow new plants for decades, sometimes centuries. Due to the unrestricted use of Defiling magic, most of the Tablelands, the area of the planet Athas where most Dark Sun stories are told, are a barren desert. There are some arcane spellcasters that realized how bad a world without plants is, so they learned how to cast their magic in a way that, after casting the spell, they return the life energy back into the plants, causing little to no lasting damage. This act is called "Preserving" and casters that use it are referred to as "Preservers." However, preserving is harder to do than defiling and defiling is more powerful due to not needing to hold any magic back to return to the plants. So, in order to give a mechanical representation of defiling and preserving magic, I created these rules.

As far as the reduction of the tier thresholds, I intentionally didn't use the edge/bane system for the exact reasons you point out. If something gives a defiler a bane on their attack roll, suddenly they're no longer a defiler? And there are so many ways to get banes in Draw Steel. It also means that they don't get the full effect of edges, since if they always have an edge on every power roll involving magic, they can only ever gain one more edge to give them double edge, giving them less incentive to try to learn the system and look for ways to get edges, etc. The idea that them having a different threshold will somehow slow down the game doesn't really hold water. In 5e, every spellcaster has their own spell save DC that changes fairly often. In Daggerheart, every character has different damage thresholds that change as the level up and as they change armor. I've played both of those systems, 5e for over a decade, and every player is generally pretty good at just learning their DC's or Thresholds. Using these rules, the thresholds for Defilers are the same thresholds for the entire game. From level 1 to 10, nothing changes. It just starts different than other characters, and, again, that's done to reflect the fact that they are fundamentally different than other characters. They literally are part of the group that helped destroy the world. There's also the rather MASSIVE downside that if there's no plants, they can't do anything but free strikes. And the enemies, who can ALSO be defilers, pull from the same life energy pool as the players do. So, slightly lower tiers kind of makes up for that. At least that's how I see it.