The point of this concept is that the ability should provide some sort of limitation to the pokemon and that limitation must be worked around (not ignored). Defeatist says that you have half attack and special attack below half HP, but still full speed and full bulk, so a pokemon that can still make progress under half and has the ability to heal up would completely fulfill the concept; "Defective Ability is an Actualization concept; aiming to create a Pokemon that... works with an ability that would be considered bad on most pokemon."The concept clearly spells out that abilities that the only real qualifying factor here is that the ability itself should be considered bad on most pokemon, that the ability should limit the mon in some way, and the pokemon should be set up in such a way that it can make the most of the ability and work around its limitations.
At the same time, I'd consider completely ignoring the ability to be in violation of the concept, if not in writing, then in spirit. A slow special tank with Slow Start is effectively working with NCA, which we ruled out in an earlier stage; a wall with defeatist is similarly working with NCA, which was ruled out. Just because there are interpretations of these abilities that are effectively NCA doesn't mean that they are NCA in practice.
I am again throwing my support behind generally bad abilities (color change / defeatist / emergency exit / klutz / normalize / slow start / stall / truant) and heavily double-edged abilities (mimicry, perish body) (those where the opponent has primary control). That said, of the generally bad abilities I would argue that Truant probably does not have enough depth to run the CAP on. There are some other double-edged abilities (Weak Armor, Gorilla Tactics, etc), but I think they generally provide an overwhelmingly positive upside, and the downside is often just either ignored, or irrelevant by the time it comes into play.
Edit:
I don't wanna make another post in response, but here's my spiel.
There's 2 situations where Mimicry has use:
1) Running with your own terrain setter:
In this case there's a fairly simple question; what does this realistically offer to a team over either a well, better terrain abuser (how can you abuse harder than Raichu-A, Kartana, or Regieleki), which could have an ability that caters to the terrain, or just well, Regieleki or Kartana stuff, and how much do you want to do strict type stacking (this is not a good strategy a lot of the time, especially for Psychic, Grass, and Electric types). Because of the problems inherent here, I am comfortable with ignoring this aspect.
2) Mimicry changing typing to match terrain
This is realistically where the ability will come up most. The issue here is that you do not have control over your own typing. From using Stunfisk-G in NU (which admittedly has an amazing base typing), the situation where you would love to be your original typing, and have your original stabs came up a lot. Your opponent being able to specifically go to their setter to screw over your defensive typing is a huge factor. Eg if you want to check a flying type (fairly common) with Stunfisk-G, they can simply go to Thwackey and then switch to the flying type to beat you. Because of the lack of control here this absolutely is a bad ability. It definitely has some upsides; guaranteed resisting Koko's and Rillaboom's STAB moves is a large benefit, its just that its outweighed by its negatives (guaranteed being weak to Rillaboom's U-turn hurts).